From 553f1e9149169cef18b01e761bb6740abb15b9fb Mon Sep 17 00:00:00 2001
From: Linus Rath <139418639+rathlinus@users.noreply.github.com>
Date: Wed, 28 Oct 2026 02:19:06 +0100
Subject: [PATCH] feat: add a switch that stops all third-party traffic #903

---
 app/(main)/admin/_pages/telemetry.tsx         |   8 +-
 app/(main)/admin/_pages/version.tsx           |   6 +
 app/api/admin/config/route.ts                 |  12 +
 app/api/admin/marketplace/[slug]/route.ts     |   5 +
 app/api/admin/marketplace/route.ts            |   8 +
 app/api/admin/marketplace/updates/route.ts    |   9 +
 app/api/admin/telemetry/route.ts              |   6 +
 app/api/admin/version/route.ts                |   2 +
 app/api/config/route.ts                       |   3 +
 app/api/connector/capabilities/route.ts       |   3 +-
 app/api/favicon/route.ts                      |   5 +
 app/api/plugin-proxy/route.ts                 |   3 +
 app/api/plugins/route.ts                      |   7 +-
 app/api/translate/route.ts                    |   5 +
 components/calendar/event-detail-popover.tsx  |   7 +-
 components/push-notification-prompt.tsx       |   3 +
 components/settings/message-list-settings.tsx |   5 +-
 components/settings/notification-settings.tsx |  12 +-
 components/settings/plugins-settings.tsx      |   5 +-
 components/ui/__tests__/avatar.test.tsx       |  14 +
 components/ui/avatar.tsx                      |  10 +-
 hooks/use-config.ts                           |   7 +
 lib/__tests__/favicon-route-inflight.test.ts  |   6 +-
 lib/__tests__/fixtures/config.ts              |   1 +
 lib/__tests__/no-third-party-client.test.ts   | 123 ++++++++
 lib/__tests__/no-third-party-server.test.ts   | 273 ++++++++++++++++++
 lib/__tests__/plugin-sandbox-loader.test.ts   |  19 ++
 lib/admin/csp-frame-origins.ts                |   7 +
 lib/admin/marketplace-directory.ts            |  21 +-
 lib/admin/third-party-mode.ts                 |  37 +++
 lib/admin/types.ts                            |   7 +
 lib/config-validation.ts                      |   1 +
 lib/lite-config.ts                            |   2 +
 lib/outgoing-hosts.ts                         | 255 ++++++++++++++++
 lib/plugin-sandbox/host-api.ts                |   6 +
 lib/plugin-sandbox/loader.ts                  |   8 +
 lib/push-relays.ts                            |  18 +-
 lib/telemetry/sender.ts                       |   2 +
 lib/telemetry/state.ts                        |   9 +-
 lib/third-party-mode.ts                       |  26 ++
 lib/version-check/sender.ts                   |   9 +
 lib/web-push.ts                               |  16 +-
 locales/de/common.json                        |   6 +
 locales/en/common.json                        |   6 +
 scripts/lite/build.mjs                        |   1 +
 scripts/lite/lib.mjs                          |   4 +
 46 files changed, 978 insertions(+), 30 deletions(-)
 create mode 100644 lib/__tests__/no-third-party-client.test.ts
 create mode 100644 lib/__tests__/no-third-party-server.test.ts
 create mode 100644 lib/admin/third-party-mode.ts
 create mode 100644 lib/outgoing-hosts.ts
 create mode 100644 lib/third-party-mode.ts

diff --git a/app/(main)/admin/_pages/telemetry.tsx b/app/(main)/admin/_pages/telemetry.tsx
index a745ac45c..796a1564d 100644
--- a/app/(main)/admin/_pages/telemetry.tsx
+++ b/app/(main)/admin/_pages/telemetry.tsx
@@ -15,7 +15,7 @@ import { cn } from '@/lib/utils';
 
 interface TelemetryStatus {
   consent: 'pending' | 'on' | 'off';
-  consentSource: 'env' | 'file';
+  consentSource: 'no-third-party' | 'env' | 'file';
   endpoint: string;
   defaultEndpoint: string;
   consentedAt: string | null;
@@ -132,14 +132,16 @@ export function TelemetryPage() {
     );
   }
 
-  const envOverridden = status.consentSource === 'env';
+  const noThirdParty = status.consentSource === 'no-third-party';
+  const envOverridden = status.consentSource === 'env' || noThirdParty;
   const isOn = status.consent === 'on';
 
   const consentDescription = [
     status.consent === 'pending' && 'Initialising - no heartbeats sent yet.',
     status.consent === 'on' && 'Heartbeats are enabled. Thanks for helping us improve!',
     status.consent === 'off' && 'Heartbeats are off (default).',
-    envOverridden && 'Locked by the BULWARK_TELEMETRY env var.',
+    noThirdParty && 'Locked off: no third-party traffic is on (Organisation > Security).',
+    envOverridden && !noThirdParty && 'Locked by the BULWARK_TELEMETRY env var.',
   ].filter(Boolean).join(' ');
 
   return (
diff --git a/app/(main)/admin/_pages/version.tsx b/app/(main)/admin/_pages/version.tsx
index e9dcd6d0c..7a700786a 100644
--- a/app/(main)/admin/_pages/version.tsx
+++ b/app/(main)/admin/_pages/version.tsx
@@ -24,6 +24,7 @@ interface VersionAdminStatus {
   endpoint: string;
   defaultEndpoint: string;
   disabledByEnv: boolean;
+  disabledByNoThirdPartyTraffic?: boolean;
   lastCheckedAt: string | null;
   lastSuccessAt: string | null;
   nextScheduledAt: string | null;
@@ -206,6 +207,11 @@ export function VersionPage() {
               {data.disabledByEnv ? 'Yes' : 'No'}
             </span>
           </SettingItem>
+          {data.disabledByNoThirdPartyTraffic && (
+            <SettingItem label="Off: no third-party traffic" description="NO_THIRD_PARTY_TRAFFIC is on, so this server never asks the version endpoint. See Organisation > Security.">
+              <span className="text-sm font-medium text-amber-600 dark:text-amber-400">Yes</span>
+            </SettingItem>
+          )}
         </SettingsSection>
       </div>
     </div>
diff --git a/app/api/admin/config/route.ts b/app/api/admin/config/route.ts
index 2981b5b8a..33e3cd203 100644
--- a/app/api/admin/config/route.ts
+++ b/app/api/admin/config/route.ts
@@ -10,6 +10,7 @@ import { logger } from '@/lib/logger';
 import { hasSessionSecret } from '@/lib/auth/session-secret';
 import { configReadOnlyResponse } from '@/lib/admin/config-read-only-response';
 import { checkExtensionDirectoryUrl } from '@/lib/admin/directory-url';
+import { parseNoThirdPartyEnv } from '@/lib/third-party-mode';
 
 // Strings that count as "no real secret configured" - used so the dashboard
 // can warn about a placeholder session secret without us ever returning the
@@ -99,6 +100,17 @@ export async function PATCH(request: NextRequest) {
       if (problem) return NextResponse.json({ error: problem }, { status: 400 });
     }
 
+    // The no-third-party switch is fail-closed: an "on" from the environment
+    // can't be undone from the panel (lib/admin/third-party-mode.ts).
+    if ('noThirdPartyTraffic' in updates) {
+      if (typeof updates.noThirdPartyTraffic !== 'boolean') {
+        return NextResponse.json({ error: 'noThirdPartyTraffic must be true or false' }, { status: 400 });
+      }
+      if (!updates.noThirdPartyTraffic && parseNoThirdPartyEnv(process.env.NO_THIRD_PARTY_TRAFFIC)) {
+        return NextResponse.json({ error: 'NO_THIRD_PARTY_TRAFFIC is set in the environment; change it there.' }, { status: 409 });
+      }
+    }
+
     // Enum keys only accept their declared values; a stray string would
     // otherwise be persisted and fail closed (or open) at the read site.
     for (const [key, value] of Object.entries(updates)) {
diff --git a/app/api/admin/marketplace/[slug]/route.ts b/app/api/admin/marketplace/[slug]/route.ts
index 4df6da909..f35fd899b 100644
--- a/app/api/admin/marketplace/[slug]/route.ts
+++ b/app/api/admin/marketplace/[slug]/route.ts
@@ -12,6 +12,8 @@ import {
   directoryJson,
   getDirectoryChannel,
   getDirectoryUrl,
+  directoryOff,
+  DIRECTORY_OFF_MESSAGE,
   historyFor,
   installedExtensions,
   installedStateOf,
@@ -65,6 +67,9 @@ export async function GET(
   try {
     const result = await requireAdminAuth(request);
     if ('error' in result) return result.error;
+    if (await directoryOff()) {
+      return NextResponse.json({ error: DIRECTORY_OFF_MESSAGE, code: 'no-third-party-traffic' }, { status: 403 });
+    }
 
     const { slug } = await params;
     const requested = request.nextUrl.searchParams.get('version');
diff --git a/app/api/admin/marketplace/route.ts b/app/api/admin/marketplace/route.ts
index 50404e882..19cfb739e 100644
--- a/app/api/admin/marketplace/route.ts
+++ b/app/api/admin/marketplace/route.ts
@@ -32,6 +32,8 @@ import {
   directoryJson,
   getDirectoryChannel,
   getDirectoryUrl,
+  directoryOff,
+  DIRECTORY_OFF_MESSAGE,
   installedExtensions,
   installedStateOf,
   readBundleBytes,
@@ -57,6 +59,9 @@ export async function GET(request: NextRequest) {
   try {
     const result = await requireAdminAuth(request);
     if ('error' in result) return result.error;
+    if (await directoryOff()) {
+      return NextResponse.json({ error: DIRECTORY_OFF_MESSAGE, code: 'no-third-party-traffic' }, { status: 403 });
+    }
 
     const directoryUrl = await getDirectoryUrl();
     const query: Record<string, string | undefined> = {};
@@ -105,6 +110,9 @@ export async function POST(request: NextRequest) {
   try {
     const result = await requireAdminAuth(request);
     if ('error' in result) return result.error;
+    if (await directoryOff()) {
+      return NextResponse.json({ error: DIRECTORY_OFF_MESSAGE, code: 'no-third-party-traffic' }, { status: 403 });
+    }
 
     const ip = getClientIP(request);
     const { slug, version, type, force } = await request.json();
diff --git a/app/api/admin/marketplace/updates/route.ts b/app/api/admin/marketplace/updates/route.ts
index 52b14e8cd..7d2643452 100644
--- a/app/api/admin/marketplace/updates/route.ts
+++ b/app/api/admin/marketplace/updates/route.ts
@@ -8,6 +8,8 @@ import {
   directoryJson,
   getDirectoryChannel,
   getDirectoryUrl,
+  directoryOff,
+  DIRECTORY_OFF_MESSAGE,
   installedExtensions,
   v2Url,
   type DirectoryAdvisory,
@@ -43,6 +45,13 @@ export async function GET(request: NextRequest) {
     const list = [...installed].map(([id, e]) => ({ id, ...e }));
     const ask = list.filter((e) => askDirectoryAbout(e.source)).map((e) => ({ slug: e.id, version: e.version }));
 
+    // The installed list is still useful with the official directory off (#903).
+    if (await directoryOff()) {
+      return NextResponse.json(
+        { installed: list, ...EMPTY, directoryUrl: null, channel: getDirectoryChannel(), appVersion: APP_VERSION || null, error: DIRECTORY_OFF_MESSAGE },
+        { headers: { 'Cache-Control': 'no-store' } },
+      );
+    }
     const directoryUrl = await getDirectoryUrl();
     const channel = getDirectoryChannel();
     let answer: UpdatesAnswer = EMPTY;
diff --git a/app/api/admin/telemetry/route.ts b/app/api/admin/telemetry/route.ts
index 192e64b31..daa5c19f8 100644
--- a/app/api/admin/telemetry/route.ts
+++ b/app/api/admin/telemetry/route.ts
@@ -81,6 +81,12 @@ export async function POST(request: NextRequest) {
           { status: 409 },
         );
       }
+      if (source === 'no-third-party') {
+        return NextResponse.json(
+          { error: 'telemetry is off while NO_THIRD_PARTY_TRAFFIC is on' },
+          { status: 409 },
+        );
+      }
       if (body.consent !== 'on' && body.consent !== 'off') {
         return NextResponse.json({ error: 'consent must be "on" or "off"' }, { status: 400 });
       }
diff --git a/app/api/admin/version/route.ts b/app/api/admin/version/route.ts
index 2c63a7b33..004db83d2 100644
--- a/app/api/admin/version/route.ts
+++ b/app/api/admin/version/route.ts
@@ -9,6 +9,7 @@ import {
   disabledByEnv,
   DEFAULT_VERSION_ENDPOINT,
 } from '@/lib/version-check';
+import { isNoThirdPartyTrafficOn } from '@/lib/admin/third-party-mode';
 
 /**
  * GET /api/admin/version
@@ -27,6 +28,7 @@ export async function GET(request: NextRequest) {
         endpoint: effectiveEndpoint(state),
         defaultEndpoint: DEFAULT_VERSION_ENDPOINT,
         disabledByEnv: disabledByEnv(),
+        disabledByNoThirdPartyTraffic: await isNoThirdPartyTrafficOn(),
         lastCheckedAt: state.lastCheckedAt,
         lastSuccessAt: state.lastSuccessAt,
         nextScheduledAt: state.nextScheduledAt,
diff --git a/app/api/config/route.ts b/app/api/config/route.ts
index 1faf393c3..d2293318d 100644
--- a/app/api/config/route.ts
+++ b/app/api/config/route.ts
@@ -11,6 +11,7 @@ import {
   pickRequestHost,
   type BrandingOverrideKey,
 } from '@/lib/admin/domain-branding';
+import { isNoThirdPartyTrafficOn } from '@/lib/admin/third-party-mode';
 
 /**
  * Runtime configuration endpoint
@@ -98,6 +99,8 @@ export async function GET(request: NextRequest) {
       embeddedMode: !!allowedFrameAncestors && allowedFrameAncestors !== "'none'",
       parentOrigin: configManager.get<string>('parentOrigin', ''),
       sourceCodeUrl: httpUrlOrEmpty(configManager.get<string>('sourceCodeUrl', '')),
+      // Browser-side switches of lib/outgoing-hosts.ts read this (#903).
+      noThirdPartyTraffic: await isNoThirdPartyTrafficOn(),
     },
     {
       // Runtime settings can change without new asset URLs. Prevent stale
diff --git a/app/api/connector/capabilities/route.ts b/app/api/connector/capabilities/route.ts
index 4ff6fd1a1..2235298b9 100644
--- a/app/api/connector/capabilities/route.ts
+++ b/app/api/connector/capabilities/route.ts
@@ -1,5 +1,6 @@
 import { NextResponse } from 'next/server';
 import { configManager } from '@/lib/admin/config-manager';
+import { isNoThirdPartyTrafficOn } from '@/lib/admin/third-party-mode';
 import { TARGET_NAMES } from '@/lib/connector/registry';
 
 /**
@@ -28,7 +29,7 @@ import { TARGET_NAMES } from '@/lib/connector/registry';
 export async function GET() {
   await configManager.ensureLoaded();
 
-  if (!configManager.get<boolean>('connectorEnabled', true)) {
+  if (!configManager.get<boolean>('connectorEnabled', true) || await isNoThirdPartyTrafficOn()) {
     return new NextResponse(null, { status: 404 });
   }
 
diff --git a/app/api/favicon/route.ts b/app/api/favicon/route.ts
index 7dc48fab8..311757fbe 100644
--- a/app/api/favicon/route.ts
+++ b/app/api/favicon/route.ts
@@ -1,4 +1,5 @@
 import { NextRequest, NextResponse } from 'next/server';
+import { isNoThirdPartyTrafficOn, thirdPartyRefusal } from '@/lib/admin/third-party-mode';
 
 // In-memory LRU cache: domain -> { data, contentType, fetchedAt }
 const CACHE_MAX_SIZE = 1000;
@@ -439,6 +440,10 @@ function evictOldest() {
 }
 
 export async function GET(request: NextRequest) {
+  // Sender icons come from DuckDuckGo: off, before anything else, while the
+  // server must not contact outside services (#903).
+  if (await isNoThirdPartyTrafficOn()) return thirdPartyRefusal('Sender icons');
+
   const domain = request.nextUrl.searchParams.get('domain');
 
   if (!domain || !isValidDomain(domain)) {
diff --git a/app/api/plugin-proxy/route.ts b/app/api/plugin-proxy/route.ts
index 5da089c90..2715160e7 100644
--- a/app/api/plugin-proxy/route.ts
+++ b/app/api/plugin-proxy/route.ts
@@ -1,5 +1,6 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { configManager } from '@/lib/admin/config-manager';
+import { isNoThirdPartyTrafficOn, thirdPartyRefusal } from '@/lib/admin/third-party-mode';
 import { rejectCrossOriginRequest } from '@/lib/security/same-origin';
 import { getStalwartCredentials } from '@/lib/stalwart/credentials';
 import { CREDENTIAL_ID_RE } from '@/lib/plugin-secrets/credentials';
@@ -41,6 +42,8 @@ export async function POST(request: NextRequest) {
   if (!configManager.getPolicy().features.pluginsEnabled) {
     return NextResponse.json({ error: 'Plugins are turned off', code: 'disabled' }, { status: 403 });
   }
+  // Every proxied request goes to a host outside this deployment (#903).
+  if (await isNoThirdPartyTrafficOn()) return thirdPartyRefusal('The plugin proxy');
 
   const creds = await getStalwartCredentials(request);
   if (!creds) return NextResponse.json({ error: 'Not authenticated', code: 'unauthenticated' }, { status: 401 });
diff --git a/app/api/plugins/route.ts b/app/api/plugins/route.ts
index 238ecc0e7..b5652688a 100644
--- a/app/api/plugins/route.ts
+++ b/app/api/plugins/route.ts
@@ -4,6 +4,8 @@ import { getPluginRegistry, getThemeRegistry } from '@/lib/admin/plugin-registry
 import { listDevPlugins } from '@/lib/admin/plugin-dev';
 import { configManager } from '@/lib/admin/config-manager';
 import { logger } from '@/lib/logger';
+import { isNoThirdPartyTrafficOn } from '@/lib/admin/third-party-mode';
+import { pluginDeclaresNetwork } from '@/lib/outgoing-hosts';
 
 /**
  * GET /api/plugins - Public endpoint for clients to discover server-managed plugins & themes
@@ -32,11 +34,14 @@ export async function GET(request: NextRequest) {
     // plugin without uninstalling it first.
     const devIds = new Set(devEntries.map(e => e.plugin.id));
     const installedEnabled = pluginRegistry.plugins.filter(p => p.enabled && !devIds.has(p.id));
+    // Plugins that declare network access never reach a browser while the
+    // server must stay off outside hosts (#903); the rest run as usual.
+    const noThirdParty = await isNoThirdPartyTrafficOn();
 
     const plugins = [
       ...devEntries.map(e => ({ ...e.plugin, dev: true })),
       ...installedEnabled.map(p => ({ ...p, dev: false })),
-    ].map(p => ({
+    ].filter(p => !(noThirdParty && pluginDeclaresNetwork(p))).map(p => ({
       id: p.id,
       name: p.name,
       version: p.version,
diff --git a/app/api/translate/route.ts b/app/api/translate/route.ts
index cdf729636..7507303f6 100644
--- a/app/api/translate/route.ts
+++ b/app/api/translate/route.ts
@@ -2,6 +2,7 @@ import { NextRequest, NextResponse } from 'next/server';
 import { rejectCrossOriginRequest } from '@/lib/security/same-origin';
 import { readFileEnv } from "@/lib/read-file-env";
 import { getStalwartCredentials } from '@/lib/stalwart/credentials';
+import { isNoThirdPartyTrafficOn, thirdPartyRefusal } from '@/lib/admin/third-party-mode';
 
 // Host-side proxy backing the "Translate" plugin (manifest apiPostPaths:
 // ["/api/translate"]). The plugin slot iframe POSTs { text, target, source,
@@ -284,6 +285,10 @@ export async function POST(request: NextRequest) {
   const target = typeof body.target === 'string' && body.target.trim() ? body.target.trim() : 'en';
   const source = typeof body.source === 'string' && body.source.trim() ? body.source.trim() : 'auto';
   const provider: Provider = body.provider === 'libretranslate' ? 'libretranslate' : 'mymemory';
+  // MyMemory is a public service; the admin's own LibreTranslate is not (#903).
+  if (provider === 'mymemory' && await isNoThirdPartyTrafficOn()) {
+    return thirdPartyRefusal('Translation through MyMemory');
+  }
 
   if (!text) {
     return NextResponse.json({ error: 'No text to translate' }, { status: 400 });
diff --git a/components/calendar/event-detail-popover.tsx b/components/calendar/event-detail-popover.tsx
index 4ab0eba77..0df3ef58d 100644
--- a/components/calendar/event-detail-popover.tsx
+++ b/components/calendar/event-detail-popover.tsx
@@ -25,6 +25,7 @@ import { canUserRsvp, getEventEditability, type EditabilityContext } from "@/lib
 import { useFormatEventDate } from "@/hooks/use-format-event-date";
 import { useContactNameResolver } from "@/hooks/use-contact-name-resolver";
 import { findMeetingLink, locationAction, mapsUrl, primaryLocationName } from "@/lib/event-links";
+import { mapsLinksAllowed } from "@/lib/outgoing-hosts";
 import { PluginSlot } from "@/components/plugins/plugin-slot";
 import { eventToView } from "@/lib/plugin-projection";
 
@@ -160,7 +161,9 @@ export function EventDetailPopover({
   const locationHref = useMemo(() => {
     if (!locationName) return null;
     const action = locationAction(locationName, meeting);
-    return action.kind === "url" ? action.uri : mapsUrl(action.query);
+    if (action.kind === "url") return action.uri;
+    // No Google Maps link while third-party traffic is off (#903).
+    return mapsLinksAllowed() ? mapsUrl(action.query) : null;
   }, [locationName, meeting]);
   const [locationCopied, setLocationCopied] = useState(false);
   const copiedTimer = useRef<ReturnType<typeof setTimeout> | null>(null);
@@ -493,7 +496,7 @@ export function EventDetailPopover({
               href={locationHref ?? undefined}
               target="_blank"
               rel="noreferrer"
-              className="text-sm text-primary hover:underline min-w-0 break-words"
+              className={cn("text-sm min-w-0 break-words", locationHref ? "text-primary hover:underline" : "text-foreground")}
               title={locationName}
             >
               {/^https?:\/\//i.test(locationName)
diff --git a/components/push-notification-prompt.tsx b/components/push-notification-prompt.tsx
index da584f836..47b5a70c3 100644
--- a/components/push-notification-prompt.tsx
+++ b/components/push-notification-prompt.tsx
@@ -121,6 +121,7 @@ export function PushNotificationPrompt() {
   );
 
   const accountId = client?.getAccountId() ?? null;
+  // Empty when no relay may be used (third-party traffic off, no own relay).
   const relayBaseUrl = resolveActiveRelayUrl(policy, userPushRelayUrl);
 
   const dismissForSession = useCallback((id: string) => {
@@ -252,6 +253,7 @@ export function PushNotificationPrompt() {
       || wasDismissed(accountId)
       || !isWebPushSupported()
       || Notification.permission === "denied"
+      || !relayBaseUrl
     ) {
       return;
     }
@@ -284,6 +286,7 @@ export function PushNotificationPrompt() {
     pathname,
     policyLoaded,
     pwaPromptVisible,
+    relayBaseUrl,
     sessionDismissedAccountIds,
   ]);
 
diff --git a/components/settings/message-list-settings.tsx b/components/settings/message-list-settings.tsx
index 2f730c958..7d47fc158 100644
--- a/components/settings/message-list-settings.tsx
+++ b/components/settings/message-list-settings.tsx
@@ -13,6 +13,7 @@ import { useFaviconRefreshStore } from '@/stores/favicon-refresh-store';
 import { toast } from '@/stores/toast-store';
 import { cn } from '@/lib/utils';
 import { IS_LITE } from '@/lib/lite';
+import { senderFaviconsAllowed } from '@/lib/outgoing-hosts';
 import { SettingsSection, SettingItem, Select, ToggleSwitch } from './settings-section';
 import { MessageListOrderSettings } from './message-list-order-settings';
 import { Button } from '@/components/ui/button';
@@ -216,13 +217,13 @@ export function MessageListSettings() {
 
       {/* Sender favicons come from the /api/favicon proxy, absent in the static Lite build (components/ui/avatar.tsx). */}
       <SettingsSection id="senders" title={tPage('sections.senders')}>
-        {!IS_LITE && (
+        {!IS_LITE && senderFaviconsAllowed() && (
           <SettingItem label={tAdvanced('sender_favicons.label')} description={tAdvanced('sender_favicons.description')}>
             <ToggleSwitch checked={senderFavicons} onChange={(checked) => updateSetting('senderFavicons', checked)} />
           </SettingItem>
         )}
 
-        {!IS_LITE && senderFavicons && (
+        {!IS_LITE && senderFaviconsAllowed() && senderFavicons && (
           <SettingItem nested label={tAdvanced('sender_favicons.refresh_label')} description={tAdvanced('sender_favicons.refresh_description')}>
             <Button
               variant="outline"
diff --git a/components/settings/notification-settings.tsx b/components/settings/notification-settings.tsx
index ebc19653a..a3285ad10 100644
--- a/components/settings/notification-settings.tsx
+++ b/components/settings/notification-settings.tsx
@@ -29,6 +29,7 @@ import {
 } from '@/lib/web-push';
 import type { PushDevice } from '@/lib/web-push';
 import { IS_LITE } from '@/lib/lite';
+import { noThirdPartyTraffic } from '@/lib/third-party-mode';
 import {
   resolveActiveRelayUrl,
   resolvePushRelayOptions,
@@ -50,6 +51,7 @@ type DevicesState =
 export function NotificationSettings() {
   const t = useTranslations('settings.notifications');
   const tAppearance = useTranslations('settings.appearance');
+  const tThirdParty = useTranslations('third_party');
   const {
     emailNotificationsEnabled,
     emailNotificationSound,
@@ -83,6 +85,8 @@ export function NotificationSettings() {
   const relayOptions = resolvePushRelayOptions(policy);
   const activeRelayUrl = resolveActiveRelayUrl(policy, pushRelayUrl);
   const relayChoiceFixed = pushRelayLocked || relayOptions.length < 2;
+  // Third-party traffic off and no relay of the organisation's own (#903).
+  const noRelay = !activeRelayUrl;
   const activeRelayLabel =
     relayOptions.find((option) => option.url === activeRelayUrl)?.label ?? activeRelayUrl;
 
@@ -140,6 +144,8 @@ export function NotificationSettings() {
   // components/service-worker-registration.tsx), not a browser limitation.
   const statusDescription = pushStatus.kind === 'unsupported'
     ? (IS_LITE ? t('push.status_lite') : `${t('push.status_unsupported')} ${t('push.ios_hint')}`)
+    : noRelay && !pushEnabled
+      ? tThirdParty('push_needs_relay')
     : busy
       ? t('push.status_busy')
       : pushEnabled
@@ -420,7 +426,7 @@ export function NotificationSettings() {
             <ToggleSwitch
               checked={pushEnabled}
               onChange={(checked) => void (checked ? handleEnablePush() : handleDisablePush())}
-              disabled={busy || pushStatus.kind === 'unsupported' || !client}
+              disabled={busy || pushStatus.kind === 'unsupported' || !client || (noRelay && !pushEnabled)}
             />
           </div>
         </SettingItem>
@@ -432,11 +438,11 @@ export function NotificationSettings() {
         <SettingItem
           advanced
           label={t('push.relay_label')}
-          description={pushRelayLocked ? t('push.relay_locked_desc') : t('push.relay_desc')}
+          description={pushRelayLocked ? t('push.relay_locked_desc') : noThirdPartyTraffic() ? tThirdParty('relay_desc') : t('push.relay_desc')}
           locked={pushRelayLocked}
         >
           {relayChoiceFixed ? (
-            <span className="text-sm text-muted-foreground">{activeRelayLabel}</span>
+            <span className="text-sm text-muted-foreground">{noRelay ? tThirdParty('relay_none') : activeRelayLabel}</span>
           ) : (
             <Select
               value={activeRelayUrl}
diff --git a/components/settings/plugins-settings.tsx b/components/settings/plugins-settings.tsx
index 4c2c15df5..a909673a4 100644
--- a/components/settings/plugins-settings.tsx
+++ b/components/settings/plugins-settings.tsx
@@ -14,6 +14,7 @@ import { ConfirmDialog } from '@/components/ui/confirm-dialog';
 import { PluginPermissionList } from '@/components/plugins/plugin-permission-list';
 import type { InstalledPlugin, PluginStatus, SettingFieldSchema } from '@/lib/plugin-types';
 import { effectiveSettings } from '@/lib/plugin-settings';
+import { PLUGIN_REFUSED_ERROR } from '@/lib/outgoing-hosts';
 import { lookup } from '@/lib/plugin-sandbox/plugin-i18n';
 import { forgetCredentials, listConnectedAccounts, type ConnectedAccount } from '@/lib/plugin-sandbox/connect-dialog';
 
@@ -213,12 +214,14 @@ interface PluginCardProps {
 
 function PluginCard({ plugin, isExpanded, isForceEnabled, controlsDisabled, onToggleExpand, onToggle, onRemove, onUpdateSettings }: PluginCardProps) {
   const t = useTranslations('plugins.settings');
+  const tThirdParty = useTranslations('third_party');
   const locale = useLocale();
   const isManaged = Boolean(plugin.managed);
   const awaiting = !isManaged && plugin.approvalStatus === 'pending';
   const denied = !isManaged && plugin.approvalStatus === 'denied';
   const errorText = plugin.error
-    ? ERROR_CODES.has(plugin.error) ? t(`error_${plugin.error.replace(/-/g, '_')}`) : plugin.error
+    ? plugin.error === PLUGIN_REFUSED_ERROR ? tThirdParty('plugin_refused')
+      : ERROR_CODES.has(plugin.error) ? t(`error_${plugin.error.replace(/-/g, '_')}`) : plugin.error
     : null;
 
   return (
diff --git a/components/ui/__tests__/avatar.test.tsx b/components/ui/__tests__/avatar.test.tsx
index 3c1be9ae7..30bb67127 100644
--- a/components/ui/__tests__/avatar.test.tsx
+++ b/components/ui/__tests__/avatar.test.tsx
@@ -6,6 +6,7 @@ import { useContactStore } from '@/stores/contact-store';
 import { useFaviconRefreshStore } from '@/stores/favicon-refresh-store';
 import { clearContactPhotoCache } from '@/lib/contact-photo-cache';
 import type { ContactCard } from '@/lib/jmap/types';
+import { setNoThirdPartyTraffic } from '@/lib/third-party-mode';
 
 describe('Avatar', () => {
   it('renders initials from full name', () => {
@@ -230,4 +231,17 @@ describe('Avatar', () => {
     expect(stamp).toBeGreaterThan(0);
     expect(img()?.getAttribute('src')).toBe(`/api/favicon?domain=example.cz&v=${stamp}`);
   });
+
+  // #903: no sender icons, and so no /api/favicon request, while third-party traffic is off.
+  it('shows initials instead of the sender favicon in the no-third-party mode', () => {
+    useFaviconRefreshStore.setState({ refreshedAt: 0 });
+    setNoThirdPartyTraffic(true);
+    try {
+      const { container } = render(<Avatar name="Jan" email="jan@example.cz" />);
+      expect(container.querySelector('img')).toBeNull();
+      expect(container.textContent).toBe('JA');
+    } finally {
+      setNoThirdPartyTraffic(false);
+    }
+  });
 });
diff --git a/components/ui/avatar.tsx b/components/ui/avatar.tsx
index d17545464..40f311cdb 100644
--- a/components/ui/avatar.tsx
+++ b/components/ui/avatar.tsx
@@ -9,6 +9,7 @@ import { useContactPhotoSrc } from "@/hooks/use-contact-photo";
 import { avatarHooks } from "@/lib/plugin-hooks";
 import { withBasePath } from "@/lib/browser-navigation";
 import { IS_LITE } from "@/lib/lite";
+import { sampleAvatarsAllowed, senderFaviconsAllowed } from "@/lib/outgoing-hosts";
 import { useFaviconRefreshStore, failedFaviconDomains } from "@/stores/favicon-refresh-store";
 
 const IS_DEV = process.env.NODE_ENV !== "production";
@@ -126,7 +127,7 @@ const CUSTOM_AVATARS: Record<string, string> = IS_DEV ? {
 // Mock-server-only: for personal-domain emails, deterministically pick a randomuser.me portrait.
 // Returns null for ~30% of addresses so not everyone has a photo.
 function getProfilePictureUrl(email: string, domain: string, devMode: boolean, name?: string): string | null {
-  if (!devMode) return null;
+  if (!devMode || !sampleAvatarsAllowed()) return null;
   if (!PERSONAL_DOMAINS.has(domain)) return null;
   const h = emailHash(email);
   if (h % 10 < 3) return null; // ~30% get no photo
@@ -237,12 +238,13 @@ export function Avatar({ name, email, contactPhoto, size = "md", className, disa
   };
 
   const profilePic = email && domain ? getProfilePictureUrl(email, domain, devMode, name) : null;
-  // Sender favicons come from the /api/favicon proxy, absent in the static build.
+  // Sender favicons come from the /api/favicon proxy, absent in the static
+  // build and off while third-party traffic is (#903).
   const showFavicon =
-    !IS_LITE && !disableFavicon && senderFavicons && faviconDomain && !PERSONAL_DOMAINS.has(faviconDomain) && !imgError && !domainFailed;
+    !IS_LITE && senderFaviconsAllowed() && !disableFavicon && senderFavicons && faviconDomain && !PERSONAL_DOMAINS.has(faviconDomain) && !imgError && !domainFailed;
 
   // Priority: contact photo > plugin avatar (e.g. Gravatar) > custom avatar > profile picture > company favicon > initials
-  const customAvatar = devMode && email ? CUSTOM_AVATARS[email.toLowerCase()] : null;
+  const customAvatar = devMode && email && sampleAvatarsAllowed() ? CUSTOM_AVATARS[email.toLowerCase()] : null;
   const pluginAvatar = pluginAvatarFailed ? null : pluginAvatarUrl;
   const photoSrc = resolvedContactPhoto || pluginAvatar || customAvatar || profilePic || null;
   const faviconSrc = !imgError && !domainFailed && showFavicon ? withBasePath(`/api/favicon?domain=${encodeURIComponent(faviconDomain!)}${faviconRefreshedAt ? `&v=${faviconRefreshedAt}` : ""}`) : null;
diff --git a/hooks/use-config.ts b/hooks/use-config.ts
index e69c6f650..7501df4b3 100644
--- a/hooks/use-config.ts
+++ b/hooks/use-config.ts
@@ -7,6 +7,7 @@ import type { PublicJmapServerEntry } from '@/lib/admin/jmap-servers';
 import { IS_LITE, IS_LITE_STALWART, LITE_CONFIG_PATH, withLiteBuildId } from '@/lib/lite';
 import { applyLiteConfig, liteStalwartDefaults } from '@/lib/lite-config';
 import { isConfigData } from '@/lib/config-validation';
+import { setNoThirdPartyTraffic } from '@/lib/third-party-mode';
 
 export interface ConfigData {
   appName: string;
@@ -46,6 +47,8 @@ export interface ConfigData {
   embeddedMode: boolean;
   parentOrigin: string;
   sourceCodeUrl: string;
+  /** Stay off every built-in outside host (#903; lib/outgoing-hosts.ts). */
+  noThirdPartyTraffic: boolean;
 }
 
 interface AppConfig extends ConfigData {
@@ -123,6 +126,7 @@ export async function fetchConfig(): Promise<ConfigData> {
   configPromise = (IS_LITE ? fetchLiteConfig() : fetchServerConfig())
     .then((data) => {
       configCache = data;
+      setNoThirdPartyTraffic(data.noThirdPartyTraffic === true);
       // Fetch admin policy alongside config (non-blocking)
       usePolicyStore.getState().fetchPolicy();
       return data;
@@ -181,6 +185,7 @@ export function useConfig(): AppConfig {
     embeddedMode: configCache?.embeddedMode || false,
     parentOrigin: configCache?.parentOrigin || '',
     sourceCodeUrl: configCache?.sourceCodeUrl || '',
+    noThirdPartyTraffic: configCache?.noThirdPartyTraffic ?? false,
     isLoading: !configCache,
     error: null,
   });
@@ -226,6 +231,7 @@ export function useConfig(): AppConfig {
         embeddedMode: configCache.embeddedMode,
         parentOrigin: configCache.parentOrigin,
         sourceCodeUrl: configCache.sourceCodeUrl,
+        noThirdPartyTraffic: configCache.noThirdPartyTraffic,
         isLoading: false,
         error: null,
       });
@@ -272,6 +278,7 @@ export function useConfig(): AppConfig {
           embeddedMode: data.embeddedMode,
           parentOrigin: data.parentOrigin,
           sourceCodeUrl: data.sourceCodeUrl,
+          noThirdPartyTraffic: data.noThirdPartyTraffic,
           isLoading: false,
           error: null,
         });
diff --git a/lib/__tests__/favicon-route-inflight.test.ts b/lib/__tests__/favicon-route-inflight.test.ts
index d559581b3..88b79dc6e 100644
--- a/lib/__tests__/favicon-route-inflight.test.ts
+++ b/lib/__tests__/favicon-route-inflight.test.ts
@@ -30,8 +30,8 @@ describe('favicon route upstream concurrency', () => {
     const inFlight = Array.from({ length: 16 }, (_, i) =>
       GET(new NextRequest(`https://webmail.example/api/favicon?domain=site${i}.example`)),
     );
-    await new Promise((r) => setTimeout(r, 0));
-    expect(fetchMock).toHaveBeenCalledTimes(16);
+    // The route reads the no-third-party switch from config first (#903).
+    await vi.waitFor(() => expect(fetchMock).toHaveBeenCalledTimes(16));
 
     const over = await GET(new NextRequest('https://webmail.example/api/favicon?domain=site99.example'));
     expect(fetchMock).toHaveBeenCalledTimes(16);
@@ -42,7 +42,7 @@ describe('favicon route upstream concurrency', () => {
     for (const release of pending.splice(0)) release();
     await Promise.all(inFlight);
     const later = GET(new NextRequest('https://webmail.example/api/favicon?domain=site100.example'));
-    expect(fetchMock).toHaveBeenCalledTimes(17);
+    await vi.waitFor(() => expect(fetchMock).toHaveBeenCalledTimes(17));
     for (const release of pending.splice(0)) release();
     expect((await later).status).toBe(200);
   });
diff --git a/lib/__tests__/fixtures/config.ts b/lib/__tests__/fixtures/config.ts
index b2824f5d1..4dd018c3c 100644
--- a/lib/__tests__/fixtures/config.ts
+++ b/lib/__tests__/fixtures/config.ts
@@ -40,6 +40,7 @@ export function createConfig(overrides: Partial<ConfigData> = {}): ConfigData {
     embeddedMode: false,
     parentOrigin: '',
     sourceCodeUrl: '',
+    noThirdPartyTraffic: false,
     ...overrides,
   };
 }
diff --git a/lib/__tests__/no-third-party-client.test.ts b/lib/__tests__/no-third-party-client.test.ts
new file mode 100644
index 000000000..a74604fc5
--- /dev/null
+++ b/lib/__tests__/no-third-party-client.test.ts
@@ -0,0 +1,123 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { DEFAULT_RELAY_BASE_URL, resolveActiveRelayUrl, resolveDefaultRelayUrl, resolvePushRelayOptions } from '@/lib/push-relays';
+import { revokePushDevice, type PushDevice } from '@/lib/web-push';
+import { doGetPublicKeyFromWKD } from '@/lib/plugin-sandbox/host-api';
+import { applyLiteConfig } from '@/lib/lite-config';
+import { setNoThirdPartyTraffic, noThirdPartyTraffic, parseNoThirdPartyEnv } from '@/lib/third-party-mode';
+import { isBuiltInHost, isPlaceholderHost, pluginAllowed, pluginDeclaresNetwork } from '@/lib/outgoing-hosts';
+import type { IJMAPClient } from '@/lib/jmap/client-interface';
+
+// The browser half of the "no third-party traffic" mode (#903).
+
+const fetchSpy = vi.fn(async () => new Response('{}'));
+
+beforeEach(() => {
+  fetchSpy.mockClear();
+  vi.stubGlobal('fetch', fetchSpy);
+});
+afterEach(() => {
+  vi.unstubAllGlobals();
+  setNoThirdPartyTraffic(false);
+});
+
+const ownRelay = { label: 'Company', url: 'https://push.company.example' };
+
+describe('push relays', () => {
+  it('drop the built-in relay in the mode and keep the admin\'s', () => {
+    setNoThirdPartyTraffic(true);
+    expect(resolvePushRelayOptions({})).toEqual([]);
+    expect(resolvePushRelayOptions({ pushRelays: [ownRelay] })).toEqual([ownRelay]);
+    setNoThirdPartyTraffic(false);
+    expect(resolvePushRelayOptions({ pushRelays: [ownRelay] }).map((o) => o.url)).toEqual([DEFAULT_RELAY_BASE_URL, ownRelay.url]);
+  });
+
+  it('fall back to the admin\'s relay, or to none, instead of the built-in one', () => {
+    setNoThirdPartyTraffic(true);
+    expect(resolveDefaultRelayUrl({ pushRelays: [ownRelay] })).toBe(ownRelay.url);
+    expect(resolveDefaultRelayUrl({})).toBe('');
+    // A user's saved pick of the built-in relay is not honoured either.
+    expect(resolveActiveRelayUrl({}, DEFAULT_RELAY_BASE_URL)).toBe('');
+    expect(resolveActiveRelayUrl({ pushRelays: [ownRelay] }, DEFAULT_RELAY_BASE_URL)).toBe(ownRelay.url);
+    // A configured pushRelayUrl pointing at the built-in host doesn't sneak it back.
+    expect(resolveDefaultRelayUrl({ pushRelayUrl: DEFAULT_RELAY_BASE_URL })).toBe('');
+    setNoThirdPartyTraffic(false);
+    expect(resolveDefaultRelayUrl({})).toBe(DEFAULT_RELAY_BASE_URL);
+  });
+
+  it('never get a request to the built-in relay in the mode', async () => {
+    const client = { destroyPushSubscription: vi.fn(async () => {}) } as unknown as IJMAPClient;
+    const device: PushDevice = { id: 'p1', deviceClientId: 'abc', expires: null, types: null, isThisDevice: false, relayStatus: 'unknown' };
+
+    setNoThirdPartyTraffic(true);
+    await revokePushDevice({ client, device, relayBaseUrl: DEFAULT_RELAY_BASE_URL });
+    expect(client.destroyPushSubscription).toHaveBeenCalledWith('p1');
+    expect(fetchSpy).not.toHaveBeenCalled();
+
+    setNoThirdPartyTraffic(false);
+    await revokePushDevice({ client, device, relayBaseUrl: DEFAULT_RELAY_BASE_URL });
+    expect(fetchSpy).toHaveBeenCalledWith(`${DEFAULT_RELAY_BASE_URL}/api/push/register/abc`, { method: 'DELETE' });
+  });
+});
+
+describe('WKD key lookup', () => {
+  it('asks the recipient\'s domain only with the mode off', async () => {
+    setNoThirdPartyTraffic(true);
+    expect(await doGetPublicKeyFromWKD('joe@vendor.example')).toMatchObject({ status: 'ERROR' });
+    expect(fetchSpy).not.toHaveBeenCalled();
+
+    setNoThirdPartyTraffic(false);
+    fetchSpy.mockImplementation(async () => new Response('', { status: 404 }));
+    await doGetPublicKeyFromWKD('joe@vendor.example');
+    expect(String((fetchSpy.mock.calls[0] as unknown[])[0])).toMatch(/^https:\/\/openpgpkey\.vendor\.example\//);
+  });
+});
+
+describe('plugins', () => {
+  it('that declare network access are refused in the mode, the rest run', () => {
+    const offline = { permissions: ['email:read', 'http:post'] };
+    const fetcher = { permissions: ['http:fetch'], httpOrigins: ['https://api.vendor.example'] };
+    const proxied = { permissions: ['http:proxy'] };
+    const framer = { permissions: [], frameOrigins: ['https://www.youtube-nocookie.com'] };
+    expect([offline, fetcher, proxied, framer].map(pluginDeclaresNetwork)).toEqual([false, true, true, true]);
+
+    setNoThirdPartyTraffic(true);
+    expect([offline, fetcher, proxied, framer].map(pluginAllowed)).toEqual([true, false, false, false]);
+    setNoThirdPartyTraffic(false);
+    expect([offline, fetcher, proxied, framer].map(pluginAllowed)).toEqual([true, true, true, true]);
+  });
+});
+
+describe('config', () => {
+  it('reads noThirdPartyTraffic from the Lite config.json', () => {
+    expect(applyLiteConfig({}).noThirdPartyTraffic).toBe(false);
+    expect(applyLiteConfig({ noThirdPartyTraffic: true }).noThirdPartyTraffic).toBe(true);
+    expect(applyLiteConfig({ noThirdPartyTraffic: 'yes' }).noThirdPartyTraffic).toBe(false);
+  });
+
+  it('turns the browser switches on once the runtime config is fetched', async () => {
+    vi.resetModules();
+    const mode = await import('@/lib/third-party-mode');
+    const { createConfig } = await import('./fixtures/config');
+    fetchSpy.mockImplementation(async () => Response.json(createConfig({ noThirdPartyTraffic: true })));
+    const { fetchConfig } = await import('@/hooks/use-config');
+    await fetchConfig();
+    expect(mode.noThirdPartyTraffic()).toBe(true);
+  });
+
+  it('parses the env spellings', () => {
+    expect(['true', 'TRUE', '1', 'yes', 'on', ' on '].map(parseNoThirdPartyEnv)).toEqual([true, true, true, true, true, true]);
+    expect(['false', '0', '', 'off', 'nope', undefined].map(parseNoThirdPartyEnv)).toEqual([false, false, false, false, false, false]);
+    expect(noThirdPartyTraffic()).toBe(false);
+  });
+});
+
+describe('host helpers', () => {
+  it('know the built-in hosts and the placeholders', () => {
+    expect(isBuiltInHost('https://notifications.relay.bulwarkmail.org/')).toBe(true);
+    expect(isBuiltInHost('https://extensions.bulwarkmail.org')).toBe(true);
+    expect(isBuiltInHost('https://extensions.corp.example')).toBe(false);
+    expect(isBuiltInHost('not a url')).toBe(false);
+    expect(['mail.example.com', 'x', 'stalwart.internal:8080', '10.0.0.5', '*.vimeo.com', 'acme.test'].every(isPlaceholderHost)).toBe(true);
+    expect(['github.com', 'icons.duckduckgo.com', 'api.example.io'].some(isPlaceholderHost)).toBe(false);
+  });
+});
diff --git a/lib/__tests__/no-third-party-server.test.ts b/lib/__tests__/no-third-party-server.test.ts
new file mode 100644
index 000000000..e4b23b7ab
--- /dev/null
+++ b/lib/__tests__/no-third-party-server.test.ts
@@ -0,0 +1,273 @@
+// @vitest-environment node
+import { afterAll, afterEach, beforeAll, beforeEach, describe, expect, it, vi } from 'vitest';
+import { mkdtempSync, rmSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import path from 'node:path';
+import { NextRequest } from 'next/server';
+
+// The server half of the "no third-party traffic" mode (#903): every route
+// and scheduler that talks to a built-in outside host stops before the
+// request once the switch is on, and works as before when it is off.
+
+vi.hoisted(() => { process.env.NEXT_PUBLIC_APP_VERSION = '1.18.0'; });
+
+vi.mock('@/lib/stalwart/credentials', () => ({
+  getStalwartCredentials: vi.fn(async () => ({ serverUrl: 'https://mail.example.com', authHeader: 'Basic x', username: 'u' })),
+}));
+vi.mock('@/lib/security/same-origin', () => ({ rejectCrossOriginRequest: () => null }));
+vi.mock('@/lib/auth/instance-details', () => ({ canSeeInstanceDetails: vi.fn(async () => true) }));
+vi.mock('@/lib/admin/session', () => ({
+  requireAdminAuth: vi.fn(async () => ({ session: { sub: 'admin' } })),
+  getClientIP: () => '127.0.0.1',
+}));
+vi.mock('@/lib/admin/audit', () => ({ auditLog: vi.fn(async () => {}) }));
+
+const registryPlugins: Record<string, unknown>[] = [];
+vi.mock('@/lib/admin/plugin-registry', () => ({
+  getPluginRegistry: vi.fn(async () => ({ plugins: registryPlugins })),
+  getThemeRegistry: vi.fn(async () => ({ themes: [] })),
+  getPlugin: vi.fn(async () => null),
+  getTheme: vi.fn(async () => null),
+}));
+vi.mock('@/lib/admin/plugin-dev', () => ({ listDevPlugins: vi.fn(async () => []) }));
+
+import { configManager } from '@/lib/admin/config-manager';
+import { isNoThirdPartyTrafficOn, noThirdPartyTrafficState } from '@/lib/admin/third-party-mode';
+
+let dir: string;
+const fetchMock = vi.fn(async (_url: unknown, _init?: unknown) => new Response(new Uint8Array(64).fill(1), { headers: { 'content-type': 'image/png' } }));
+
+function calledHosts(): string[] {
+  return fetchMock.mock.calls.map(([url]) => {
+    try { return new URL(String(url)).hostname; } catch { return String(url); }
+  });
+}
+
+/** The switch as an admin sets it: config.json, read again. */
+async function setMode(on: boolean, policy: Record<string, unknown> = {}) {
+  writeFileSync(path.join(dir, 'config.json'), JSON.stringify(on ? { noThirdPartyTraffic: true } : {}));
+  writeFileSync(path.join(dir, 'policy.json'), JSON.stringify(policy));
+  await configManager.load();
+}
+
+beforeAll(() => {
+  dir = mkdtempSync(path.join(tmpdir(), 'bulwark-3p-'));
+  process.env.ADMIN_CONFIG_DIR = dir;
+  process.env.TELEMETRY_DATA_DIR = path.join(dir, 'telemetry');
+  process.env.VERSION_CHECK_DATA_DIR = path.join(dir, 'version');
+});
+afterAll(() => {
+  rmSync(dir, { recursive: true, force: true });
+  delete process.env.ADMIN_CONFIG_DIR;
+  delete process.env.TELEMETRY_DATA_DIR;
+  delete process.env.VERSION_CHECK_DATA_DIR;
+});
+beforeEach(() => {
+  fetchMock.mockClear();
+  vi.stubGlobal('fetch', fetchMock);
+  delete process.env.NO_THIRD_PARTY_TRAFFIC;
+  registryPlugins.length = 0;
+});
+afterEach(() => {
+  vi.unstubAllGlobals();
+  delete process.env.NO_THIRD_PARTY_TRAFFIC;
+});
+
+describe('the switch', () => {
+  it('is off by default and on from config.json', async () => {
+    await setMode(false);
+    expect(await isNoThirdPartyTrafficOn()).toBe(false);
+    expect(noThirdPartyTrafficState()).toEqual({ on: false, source: 'default' });
+    await setMode(true);
+    expect(await isNoThirdPartyTrafficOn()).toBe(true);
+    expect(noThirdPartyTrafficState()).toEqual({ on: true, source: 'admin' });
+  });
+
+  it('takes an env "on" over a config.json false, and any common spelling of on', async () => {
+    writeFileSync(path.join(dir, 'config.json'), JSON.stringify({ noThirdPartyTraffic: false }));
+    await configManager.load();
+    for (const value of ['true', '1', 'YES', 'on']) {
+      process.env.NO_THIRD_PARTY_TRAFFIC = value;
+      expect(await isNoThirdPartyTrafficOn()).toBe(true);
+      expect(noThirdPartyTrafficState()).toEqual({ on: true, source: 'env' });
+    }
+    process.env.NO_THIRD_PARTY_TRAFFIC = 'false';
+    expect(await isNoThirdPartyTrafficOn()).toBe(false);
+  });
+
+  it('is in the client config', async () => {
+    const { GET } = await import('@/app/api/config/route');
+    await setMode(true);
+    expect((await (await GET(new NextRequest('https://webmail.example/api/config'))).json()).noThirdPartyTraffic).toBe(true);
+    await setMode(false);
+    expect((await (await GET(new NextRequest('https://webmail.example/api/config'))).json()).noThirdPartyTraffic).toBe(false);
+  });
+
+  it('refuses an admin "off" while the environment says on', async () => {
+    const { PATCH } = await import('@/app/api/admin/config/route');
+    process.env.NO_THIRD_PARTY_TRAFFIC = 'true';
+    const res = await PATCH(new NextRequest('https://webmail.example/api/admin/config', {
+      method: 'PATCH', body: JSON.stringify({ noThirdPartyTraffic: false }),
+    }));
+    expect(res.status).toBe(409);
+  });
+});
+
+describe('sender favicons', () => {
+  it('answer 403 without contacting DuckDuckGo', async () => {
+    const { GET } = await import('@/app/api/favicon/route');
+    await setMode(true);
+    const res = await GET(new NextRequest('https://webmail.example/api/favicon?domain=github.com'));
+    expect(res.status).toBe(403);
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+
+  it('come from DuckDuckGo with the mode off', async () => {
+    const { GET } = await import('@/app/api/favicon/route');
+    await setMode(false);
+    await GET(new NextRequest('https://webmail.example/api/favicon?domain=gitlab.com'));
+    expect(calledHosts()).toEqual(['icons.duckduckgo.com']);
+  });
+});
+
+describe('translation', () => {
+  const post = async (body: Record<string, unknown>) => {
+    const { POST } = await import('@/app/api/translate/route');
+    return POST(new NextRequest('https://webmail.example/api/translate', { method: 'POST', body: JSON.stringify(body) }));
+  };
+
+  it('refuses MyMemory before any request', async () => {
+    await setMode(true);
+    const res = await post({ text: 'Hallo Welt', target: 'en', source: 'de' });
+    expect(res.status).toBe(403);
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+
+  it('still uses MyMemory with the mode off, and the admin\'s LibreTranslate with it on', async () => {
+    fetchMock.mockImplementation(async () => Response.json({ responseData: { translatedText: 'Hello world' }, translatedText: 'Hello world' }));
+    await setMode(false);
+    await post({ text: 'Hallo Welt', target: 'en', source: 'de' });
+    expect(calledHosts()).toContain('api.mymemory.translated.net');
+
+    fetchMock.mockClear();
+    process.env.LIBRETRANSLATE_URL = 'https://translate.corp.example';
+    try {
+      await setMode(true);
+      await post({ text: 'Hallo Welt', target: 'en', source: 'de', provider: 'libretranslate' });
+      expect(calledHosts()).toEqual(['translate.corp.example']);
+    } finally {
+      delete process.env.LIBRETRANSLATE_URL;
+      fetchMock.mockImplementation(async () => new Response(new Uint8Array(64).fill(1), { headers: { 'content-type': 'image/png' } }));
+    }
+  });
+});
+
+describe('connector', () => {
+  it('answers 404 in the mode', async () => {
+    const { GET } = await import('@/app/api/connector/capabilities/route');
+    await setMode(false);
+    expect((await GET()).status).toBe(200);
+    await setMode(true);
+    expect((await GET()).status).toBe(404);
+  });
+});
+
+describe('telemetry', () => {
+  it('is off whatever the saved consent says, and send-now sends nothing', async () => {
+    const { effectiveConsent, saveState, loadState, sendOnce } = await import('@/lib/telemetry');
+    await setMode(false);
+    await saveState({ ...(await loadState()), consent: 'on' });
+    expect((await effectiveConsent()).consent).toBe('on');
+
+    await setMode(true);
+    expect(await effectiveConsent()).toMatchObject({ consent: 'off', source: 'no-third-party' });
+    expect((await sendOnce({ reason: 'test' })).ok).toBe(false);
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+});
+
+describe('version check', () => {
+  it('asks the version endpoint only with the mode off', async () => {
+    const { checkOnce } = await import('@/lib/version-check');
+    fetchMock.mockImplementation(async () => Response.json({ latest: '1.18.0', severity: 'none' }));
+    try {
+      await setMode(true);
+      expect(await checkOnce()).toMatchObject({ ok: false, error: 'off: no third-party traffic' });
+      expect(fetchMock).not.toHaveBeenCalled();
+
+      await setMode(false);
+      await checkOnce();
+      expect(calledHosts()).toEqual(['version.telemetry.bulwarkmail.org']);
+    } finally {
+      fetchMock.mockImplementation(async () => new Response(new Uint8Array(64).fill(1), { headers: { 'content-type': 'image/png' } }));
+    }
+  });
+});
+
+describe('extension directory', () => {
+  it('refuses the official directory in the mode and keeps a self-hosted one', async () => {
+    const { directoryOff } = await import('@/lib/admin/marketplace-directory');
+    const { GET } = await import('@/app/api/admin/marketplace/route');
+    await setMode(true);
+    expect(await directoryOff()).toBe(true);
+    const res = await GET(new NextRequest('https://webmail.example/api/admin/marketplace'));
+    expect(res.status).toBe(403);
+    expect(fetchMock).not.toHaveBeenCalled();
+
+    writeFileSync(path.join(dir, 'config.json'), JSON.stringify({ noThirdPartyTraffic: true, extensionDirectoryUrl: 'https://extensions.corp.example' }));
+    await configManager.load();
+    expect(await directoryOff()).toBe(false);
+
+    await setMode(false);
+    expect(await directoryOff()).toBe(false);
+  });
+
+  it('keeps the installed list of the updates view', async () => {
+    const { GET } = await import('@/app/api/admin/marketplace/updates/route');
+    await setMode(true);
+    const res = await GET(new NextRequest('https://webmail.example/api/admin/marketplace/updates'));
+    expect(res.status).toBe(200);
+    expect((await res.json()).error).toMatch(/official extension directory is off/);
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+});
+
+describe('plugins', () => {
+  const plugins = [
+    { id: 'offline', name: 'Offline', version: '1.0.0', enabled: true, permissions: ['email:read'] },
+    { id: 'fetcher', name: 'Fetcher', version: '1.0.0', enabled: true, permissions: ['http:fetch'], httpOrigins: ['https://api.vendor.example'] },
+    { id: 'proxied', name: 'Proxied', version: '1.0.0', enabled: true, permissions: ['http:proxy'] },
+    { id: 'framer', name: 'Framer', version: '1.0.0', enabled: true, permissions: [], frameOrigins: ['https://www.youtube-nocookie.com'] },
+  ];
+
+  it('that declare network access never reach a browser in the mode', async () => {
+    registryPlugins.push(...plugins);
+    const { GET } = await import('@/app/api/plugins/route');
+    await setMode(false);
+    const off = await (await GET(new NextRequest('https://webmail.example/api/plugins'))).json();
+    expect(off.plugins.map((p: { id: string }) => p.id)).toEqual(['offline', 'fetcher', 'proxied', 'framer']);
+    await setMode(true);
+    const on = await (await GET(new NextRequest('https://webmail.example/api/plugins'))).json();
+    expect(on.plugins.map((p: { id: string }) => p.id)).toEqual(['offline']);
+  });
+
+  it('get no outside frames in the CSP', async () => {
+    registryPlugins.push(...plugins);
+    const { getEnabledPluginFrameOrigins, invalidateFrameOriginsCache } = await import('@/lib/admin/csp-frame-origins');
+    await setMode(false);
+    invalidateFrameOriginsCache();
+    expect(await getEnabledPluginFrameOrigins()).toEqual(['https://www.youtube-nocookie.com']);
+    await setMode(true);
+    invalidateFrameOriginsCache();
+    expect(await getEnabledPluginFrameOrigins()).toEqual([]);
+  });
+
+  it('can\'t use the credential proxy', async () => {
+    const { POST } = await import('@/app/api/plugin-proxy/route');
+    await setMode(true, { features: { pluginsEnabled: true } });
+    const res = await POST(new NextRequest('https://webmail.example/api/plugin-proxy?plugin=proxied&credential=main&url=https%3A%2F%2Fapi.vendor.example%2F', { method: 'POST' }));
+    expect(res.status).toBe(403);
+    expect((await res.json()).code).toBe('no-third-party-traffic');
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+});
diff --git a/lib/__tests__/plugin-sandbox-loader.test.ts b/lib/__tests__/plugin-sandbox-loader.test.ts
index 8a2715553..ddb2fc49b 100644
--- a/lib/__tests__/plugin-sandbox-loader.test.ts
+++ b/lib/__tests__/plugin-sandbox-loader.test.ts
@@ -102,6 +102,25 @@ function lastStatus() {
   return statuses[statuses.length - 1];
 }
 
+describe('loadSandboxedPlugin in the no-third-party mode (#903)', () => {
+  it('refuses a plugin that declares network access and runs one that does not', async () => {
+    const { setNoThirdPartyTraffic } = await import('../third-party-mode');
+    mocks.getCode.mockResolvedValue(CODE);
+    setNoThirdPartyTraffic(true);
+    try {
+      await loadSandboxedPlugin(plugin({ permissions: ['http:fetch'], httpOrigins: ['https://api.vendor.example'] }));
+      expect(lastStatus()).toMatchObject({ id: 'managed-plugin', status: 'error', error: 'third-party-off' });
+      expect(mocks.createBackgroundInstance).not.toHaveBeenCalled();
+      expect(getActive('managed-plugin')).toBeUndefined();
+
+      await loadSandboxedPlugin(plugin({ permissions: ['email:read'] }));
+      expect(lastStatus()).toMatchObject({ id: 'managed-plugin', status: 'running' });
+    } finally {
+      setNoThirdPartyTraffic(false);
+    }
+  });
+});
+
 describe('loadSandboxedPlugin bundle sourcing', () => {
   it('runs a managed plugin from IndexedDB when the cached copy verifies', async () => {
     mocks.getCode.mockResolvedValue(CODE);
diff --git a/lib/admin/csp-frame-origins.ts b/lib/admin/csp-frame-origins.ts
index b28c40616..d3a8e6548 100644
--- a/lib/admin/csp-frame-origins.ts
+++ b/lib/admin/csp-frame-origins.ts
@@ -11,6 +11,7 @@
 
 import { getPluginRegistry } from './plugin-registry';
 import { sanitizeFrameOrigins } from '../extension-origins';
+import { isNoThirdPartyTrafficOn } from './third-party-mode';
 
 export {
   isValidFrameOrigin,
@@ -40,6 +41,12 @@ export async function getEnabledPluginFrameOrigins(): Promise<string[]> {
   if (now - cachedAt < CACHE_TTL_MS) return cachedOrigins;
 
   try {
+    // No outside frames at all while the server stays off outside hosts (#903).
+    if (await isNoThirdPartyTrafficOn()) {
+      cachedOrigins = [];
+      cachedAt = now;
+      return [];
+    }
     const registry = await getPluginRegistry();
     // Dev plugins (PLUGIN_DEV_DIR) are served next to the installed ones and
     // need their embeds allowed too.
diff --git a/lib/admin/marketplace-directory.ts b/lib/admin/marketplace-directory.ts
index 29b6868f8..fba212d0a 100644
--- a/lib/admin/marketplace-directory.ts
+++ b/lib/admin/marketplace-directory.ts
@@ -14,6 +14,8 @@
 import { configManager } from './config-manager';
 import { getPluginRegistry, getThemeRegistry, type ExtensionSource } from './plugin-registry';
 import { listHistory, type HistoryEntry } from './extension-history';
+import { isNoThirdPartyTrafficOn } from './third-party-mode';
+import { isBuiltInHost } from '@/lib/outgoing-hosts';
 
 export const APP_VERSION = process.env.NEXT_PUBLIC_APP_VERSION || '';
 
@@ -21,7 +23,24 @@ export type Channel = 'stable' | 'beta';
 
 export async function getDirectoryUrl(): Promise<string> {
   await configManager.ensureLoaded();
-  return configManager.get<string>('extensionDirectoryUrl') || 'https://extensions.bulwarkmail.org';
+  const url = configManager.get<string>('extensionDirectoryUrl') || 'https://extensions.bulwarkmail.org';
+  // Defense in depth: the routes ask directoryOff() first and answer 403.
+  if (isBuiltInHost(url) && await isNoThirdPartyTrafficOn()) throw new DirectoryError(DIRECTORY_OFF_MESSAGE, 403);
+  return url;
+}
+
+export const DIRECTORY_OFF_MESSAGE =
+  'The official extension directory is off: this server does not contact outside services (NO_THIRD_PARTY_TRAFFIC). Point EXTENSION_DIRECTORY_URL at a self-hosted directory to use the marketplace.';
+
+/**
+ * Whether the marketplace must not ask its directory: the no-third-party
+ * mode is on and the directory is the official one (#903). A self-hosted
+ * directory is the deployment's own and stays reachable.
+ */
+export async function directoryOff(): Promise<boolean> {
+  await configManager.ensureLoaded();
+  const url = configManager.get<string>('extensionDirectoryUrl') || 'https://extensions.bulwarkmail.org';
+  return isBuiltInHost(url) && await isNoThirdPartyTrafficOn();
 }
 
 export function getDirectoryChannel(): Channel {
diff --git a/lib/admin/third-party-mode.ts b/lib/admin/third-party-mode.ts
new file mode 100644
index 000000000..daaf9b8fd
--- /dev/null
+++ b/lib/admin/third-party-mode.ts
@@ -0,0 +1,37 @@
+import { NextResponse } from 'next/server';
+import { configManager } from './config-manager';
+import { parseNoThirdPartyEnv } from '@/lib/third-party-mode';
+
+/**
+ * The "no third-party traffic" switch (#903) on the server.
+ *
+ * It is config, not policy: instance-wide, because most of what it stops
+ * (telemetry, the version check, the extension directory) runs for no
+ * particular user. NO_THIRD_PARTY_TRAFFIC in the environment wins over
+ * config.json in one direction only: an env "on" can't be switched off from
+ * the admin panel, so a deployment that pins it stays closed.
+ */
+
+export type NoThirdPartySource = 'env' | 'admin' | 'default';
+
+export function noThirdPartyTrafficState(): { on: boolean; source: NoThirdPartySource } {
+  if (parseNoThirdPartyEnv(process.env.NO_THIRD_PARTY_TRAFFIC)) return { on: true, source: 'env' };
+  const entry = configManager.getAllWithSources().noThirdPartyTraffic;
+  const on = entry?.value === true;
+  return { on, source: entry?.source === 'admin' ? 'admin' : on ? 'env' : 'default' };
+}
+
+/** Whether every built-in outside host is off. Loads the config first. */
+export async function isNoThirdPartyTrafficOn(): Promise<boolean> {
+  if (parseNoThirdPartyEnv(process.env.NO_THIRD_PARTY_TRAFFIC)) return true;
+  await configManager.ensureLoaded();
+  return configManager.get<boolean>('noThirdPartyTraffic', false) === true;
+}
+
+/** The answer a route gives instead of contacting an outside host. */
+export function thirdPartyRefusal(what: string): NextResponse {
+  return NextResponse.json(
+    { error: `${what} is off: this server doesn't contact outside services (NO_THIRD_PARTY_TRAFFIC).`, code: 'no-third-party-traffic' },
+    { status: 403, headers: { 'Cache-Control': 'no-store' } },
+  );
+}
diff --git a/lib/admin/types.ts b/lib/admin/types.ts
index 0c779bef2..7f42af183 100644
--- a/lib/admin/types.ts
+++ b/lib/admin/types.ts
@@ -356,6 +356,13 @@ export const CONFIG_ENV_MAP: Record<string, { envVar: string; fileEnvVar?: strin
   // white-label or self-hosted deployment at its own connector.
   connectorEnabled: { envVar: 'CONNECTOR_ENABLED', type: 'boolean', defaultValue: true },
   connectorUrl: { envVar: 'CONNECTOR_URL', type: 'url', defaultValue: 'https://connector.bulwarkmail.org' },
+  // Fail-closed "no third-party traffic" mode (#903): every built-in outside
+  // host in lib/outgoing-hosts.ts is off (telemetry, version check, the
+  // built-in push relay, the official extension directory, the connector,
+  // sender favicons, MyMemory, maps links, WKD) and plugins that declare
+  // network access don't run. Instance-wide on purpose. An env "on" can't be
+  // switched off in the admin panel (lib/admin/third-party-mode.ts).
+  noThirdPartyTraffic: { envVar: 'NO_THIRD_PARTY_TRAFFIC', type: 'boolean', defaultValue: false },
   // WOPI document editing (#425). `wopiClientUrl` is the editor's base URL
   // (Collabora Online / OnlyOffice / EuroOffice, ...); discovery is fetched
   // from `<url>/hosting/discovery` unless the URL already carries a path.
diff --git a/lib/config-validation.ts b/lib/config-validation.ts
index 058af8d6a..0432f218e 100644
--- a/lib/config-validation.ts
+++ b/lib/config-validation.ts
@@ -64,6 +64,7 @@ const configValidators = {
   embeddedMode: isBoolean,
   parentOrigin: isString,
   sourceCodeUrl: isString,
+  noThirdPartyTraffic: isBoolean,
 } satisfies Validators<ConfigData>;
 
 /**
diff --git a/lib/lite-config.ts b/lib/lite-config.ts
index 17accce2a..93e1e2a4c 100644
--- a/lib/lite-config.ts
+++ b/lib/lite-config.ts
@@ -95,6 +95,8 @@ export function applyLiteConfig(raw: unknown, defaults?: { jmapServerUrl?: strin
     embeddedMode: bool(input.embeddedMode, false),
     parentOrigin: str(input.parentOrigin, ''),
     sourceCodeUrl: httpUrlOrEmpty(input.sourceCodeUrl),
+    // Same browser-side switches as the server build (lib/outgoing-hosts.ts).
+    noThirdPartyTraffic: bool(input.noThirdPartyTraffic, false),
     ...LITE_FORCED_FLAGS,
   };
 }
diff --git a/lib/outgoing-hosts.ts b/lib/outgoing-hosts.ts
new file mode 100644
index 000000000..46dca6769
--- /dev/null
+++ b/lib/outgoing-hosts.ts
@@ -0,0 +1,255 @@
+/**
+ * Every outside host Bulwark itself knows about, what talks to it, and what
+ * turns it off (#903).
+ *
+ * "Outside" means a host the deployment didn't configure: the admin's JMAP
+ * servers, push relays, OAuth issuer, WOPI editor, sidebar apps and a
+ * self-hosted extension directory are the deployment's own choice and stay
+ * reachable. Everything listed here is off while `noThirdPartyTraffic`
+ * (env NO_THIRD_PARTY_TRAFFIC) is on.
+ *
+ * lib/__tests__/outgoing-hosts.test.ts scans the source for http(s) host
+ * literals and fails on any that is neither listed here nor in
+ * NON_REQUEST_HOSTS, so a new built-in host can't slip in unnoticed; a
+ * second test turns the mode on and checks every entry's switch.
+ *
+ * Plain data and pure functions: imported by the browser, the server routes,
+ * the admin page and the tests.
+ */
+
+import { noThirdPartyTraffic } from './third-party-mode';
+
+export type OutgoingSide = 'server' | 'browser';
+
+export interface OutgoingHost {
+  /** Stable id, used by the tests and the admin list. */
+  id: string;
+  /** The host as people should read it. */
+  host: string;
+  /** How the host appears in the source after `https://`, when it differs from `host`. */
+  literals?: string[];
+  /** Who opens the connection: this server, or the user's browser. */
+  side: OutgoingSide;
+  /** What uses it, in one sentence. */
+  usedBy: string;
+  /** How to turn it off without the mode. */
+  off: string;
+  /** What the mode does instead. */
+  inMode: string;
+  /** Also present in Bulwark Lite. */
+  lite: boolean;
+}
+
+export const OUTGOING_HOSTS: readonly OutgoingHost[] = [
+  {
+    id: 'telemetry',
+    host: 'telemetry.bulwarkmail.org',
+    side: 'server',
+    usedBy: 'Anonymous daily heartbeat, sent only after an admin turned telemetry on.',
+    off: 'Off by default. BULWARK_TELEMETRY=off, or Admin > Telemetry.',
+    inMode: 'Never sent, whatever the telemetry setting says.',
+    lite: false,
+  },
+  {
+    id: 'version-check',
+    host: 'version.telemetry.bulwarkmail.org',
+    side: 'server',
+    usedBy: 'Hourly check for a newer Bulwark release, shown to admins.',
+    off: 'BULWARK_UPDATE_CHECK=off, or BULWARK_UPDATE_CHECK_URL= (empty).',
+    inMode: 'Not checked; Admin > Version says why.',
+    lite: false,
+  },
+  {
+    id: 'push-relay',
+    host: 'notifications.relay.bulwarkmail.org',
+    side: 'browser',
+    usedBy: 'Built-in Web Push relay that wakes the browser for new mail.',
+    off: 'Add your own relays under Admin > User policy > Push relays and lock the choice.',
+    inMode: 'Not offered. Push works only with a relay the admin configured.',
+    lite: false,
+  },
+  {
+    id: 'extension-directory',
+    host: 'extensions.bulwarkmail.org',
+    side: 'server',
+    usedBy: 'Marketplace browsing, installs and update checks in the admin panel.',
+    off: 'EXTENSION_DIRECTORY_URL pointing at a self-hosted directory.',
+    inMode: 'Off, unless extensionDirectoryUrl points at another host.',
+    lite: false,
+  },
+  {
+    id: 'connector',
+    host: 'connector.bulwarkmail.org',
+    side: 'server',
+    usedBy: 'Links in the docs and the directory that open this instance; the connector asks /api/connector/capabilities.',
+    off: 'CONNECTOR_ENABLED=false.',
+    inMode: '/api/connector/capabilities answers 404.',
+    lite: false,
+  },
+  {
+    id: 'favicons',
+    host: 'icons.duckduckgo.com',
+    side: 'server',
+    usedBy: 'Sender icons next to messages, fetched by /api/favicon.',
+    off: 'Users can turn sender icons off in Settings > Message list.',
+    inMode: 'Avatars show initials; /api/favicon answers 403 before any request.',
+    lite: false,
+  },
+  {
+    id: 'translate',
+    host: 'api.mymemory.translated.net',
+    side: 'server',
+    usedBy: 'Default provider of /api/translate, which the Translate plugin calls.',
+    off: 'Set LIBRETRANSLATE_URL to your own LibreTranslate and pick it in the plugin.',
+    inMode: 'Refused with 403; LibreTranslate at LIBRETRANSLATE_URL still works.',
+    lite: false,
+  },
+  {
+    id: 'maps',
+    host: 'www.google.com',
+    side: 'browser',
+    usedBy: 'Tapping the address of a calendar event opens Google Maps.',
+    off: 'Nothing else turns it off; it is a link the user taps.',
+    inMode: 'The address is plain text with a copy button.',
+    lite: true,
+  },
+  {
+    id: 'wkd',
+    host: 'openpgpkey.<recipient domain>',
+    literals: ['openpgpkey.'],
+    side: 'browser',
+    usedBy: 'OpenPGP key lookup (WKD) at the recipient\'s domain, for PGP plugins.',
+    off: 'Don\'t install a PGP plugin that looks keys up.',
+    inMode: 'The lookup is refused.',
+    lite: false,
+  },
+  {
+    id: 'dev-avatars',
+    host: 'randomuser.me',
+    literals: ['randomuser.me', 'img.freepik.com'],
+    side: 'browser',
+    usedBy: 'Sample sender photos, only with the built-in mock server (DEV_MOCK_JMAP=true).',
+    off: 'Don\'t run the mock server.',
+    inMode: 'Not shown.',
+    lite: false,
+  },
+];
+
+/**
+ * Hosts in the source that Bulwark never connects to on its own: docs links,
+ * XML namespaces, credits, examples in comments and sample text. Each is
+ * allowed only in the files named, so the same host turning up somewhere new
+ * has to be looked at again.
+ */
+export const NON_REQUEST_HOSTS: readonly { host: string; files: readonly string[]; why: string }[] = [
+  { host: 'bulwarkmail.org', why: 'docs links and comments', files: ['app/(main)/admin/_pages/telemetry.tsx', 'instrumentation.node.ts', 'lib/telemetry/state.ts', 'lib/telemetry/types.ts'] },
+  { host: 'bulwarkmail.com', why: 'JMAP capability URI', files: ['lib/jmap/client.ts'] },
+  { host: 'extensions.bulwarkmail.org', why: 'signed statement type URIs', files: ['lib/admin/directory-keys.ts'] },
+  { host: 'www.w3.org', why: 'XML namespaces', files: ['app/(main)/setup/page.tsx', 'components/ui/flag-icons.tsx', 'lib/favicon-badge.ts'] },
+  { host: 'creativecommons.org', why: 'licence metadata inside an SVG', files: ['components/ui/flag-icons.tsx'] },
+  { host: 'github.com', why: 'source code link (AGPL) and SVG metadata', files: ['components/settings/about-data-settings.tsx', 'components/ui/flag-icons.tsx'] },
+  { host: 'www.rfc-editor.org', why: 'comments', files: ['lib/jmap/client.ts'] },
+  { host: 'devguide.calconnect.org', why: 'comment', files: ['lib/jmap/client.ts'] },
+  { host: 'stalw.art', why: 'comment', files: ['app/api/auth/impersonate/route.ts'] },
+  { host: 'tabler.io', why: 'icon credit link', files: ['app/(main)/admin/_pages/policy.tsx'] },
+  { host: 'www.youtube-nocookie.com', why: 'example in a comment', files: ['lib/plugin-types.ts'] },
+  { host: 'bulwark-webmail.vercel.app', why: 'sample message text', files: ['stores/email-store.ts'] },
+];
+
+/** Folders whose host literals are sample mail content, never requests. */
+export const NON_REQUEST_PATHS: readonly string[] = [
+  'lib/demo/fixtures/',
+  'app/api/dev-jmap/',
+];
+
+const RESERVED_TLDS = new Set(['example', 'test', 'invalid', 'localhost', 'local', 'internal', 'lan']);
+
+/**
+ * Hosts that can't be anyone else's: example and reserved names, single
+ * labels, IP literals and CSP wildcards. Placeholders and tests use them.
+ */
+export function isPlaceholderHost(raw: string): boolean {
+  const host = raw.toLowerCase().replace(/:\d+$/, '').replace(/\.+$/, '');
+  if (!host.includes('.') || host.includes('*')) return true;
+  if (/^\d{1,3}(\.\d{1,3}){0,3}\.?$/.test(host)) return true;
+  const labels = host.split('.');
+  const tld = labels[labels.length - 1];
+  if (RESERVED_TLDS.has(tld)) return true;
+  return /(^|\.)example\.(com|org|net)$/.test(host);
+}
+
+/** Every host literal the registry accounts for. */
+export function registeredLiterals(): Set<string> {
+  const out = new Set<string>();
+  for (const entry of OUTGOING_HOSTS) for (const literal of entry.literals ?? [entry.host]) out.add(literal);
+  return out;
+}
+
+function hostnameOf(url: string): string | null {
+  try {
+    return new URL(url).hostname.toLowerCase();
+  } catch {
+    return null;
+  }
+}
+
+/** Whether a URL points at one of the built-in outside hosts above. */
+export function isBuiltInHost(url: string): boolean {
+  const host = hostnameOf(url);
+  if (!host) return false;
+  return OUTGOING_HOSTS.some((entry) => (entry.literals ?? [entry.host]).some((literal) => literal === host));
+}
+
+// ─── Browser-side switches ────────────────────────────────────────────────
+
+/** A push relay may be used: always, except the built-in one in the mode. */
+export function relayAllowed(url: string): boolean {
+  return !(noThirdPartyTraffic() && isBuiltInHost(url));
+}
+
+/** Sender icons through /api/favicon. */
+export function senderFaviconsAllowed(): boolean {
+  return !noThirdPartyTraffic();
+}
+
+/** The Google Maps link on an event's address. */
+export function mapsLinksAllowed(): boolean {
+  return !noThirdPartyTraffic();
+}
+
+/** OpenPGP WKD lookups at the recipient's domain. */
+export function wkdAllowed(): boolean {
+  return !noThirdPartyTraffic();
+}
+
+/** Sample photos of the mock server. */
+export function sampleAvatarsAllowed(): boolean {
+  return !noThirdPartyTraffic();
+}
+
+export interface NetworkDeclaringPlugin {
+  permissions?: readonly string[];
+  httpOrigins?: readonly string[];
+  frameOrigins?: readonly string[];
+}
+
+/**
+ * A plugin declares network access when it may fetch outside hosts
+ * (`http:fetch` with httpOrigins), use the credential proxy (`http:proxy`) or
+ * embed outside frames (frameOrigins). Same-origin `http:post` doesn't count.
+ */
+export function pluginDeclaresNetwork(plugin: NetworkDeclaringPlugin): boolean {
+  const permissions = plugin.permissions ?? [];
+  return permissions.includes('http:fetch')
+    || permissions.includes('http:proxy')
+    || (plugin.httpOrigins?.length ?? 0) > 0
+    || (plugin.frameOrigins?.length ?? 0) > 0;
+}
+
+/** Error code a refused plugin carries (shown translated in Settings > Plugins). */
+export const PLUGIN_REFUSED_ERROR = 'third-party-off';
+
+/** Whether a plugin may run in this browser. */
+export function pluginAllowed(plugin: NetworkDeclaringPlugin): boolean {
+  return !(noThirdPartyTraffic() && pluginDeclaresNetwork(plugin));
+}
diff --git a/lib/plugin-sandbox/host-api.ts b/lib/plugin-sandbox/host-api.ts
index 0790e8a09..87f72dde8 100644
--- a/lib/plugin-sandbox/host-api.ts
+++ b/lib/plugin-sandbox/host-api.ts
@@ -59,6 +59,7 @@ import { doNotificationsShow } from './host-api-notify';
 import { setSectionTitle } from './section-titles';
 import { requestPanel } from './panel-requests';
 import { doComposerAttach, doComposerGetBody, doComposerGetSelection, doComposerReplaceBody, doComposerReplaceSelection } from './host-api-composer';
+import { pluginAllowed, wkdAllowed } from '../outgoing-hosts';
 
 /**
  * Methods only callable from the privileged (same-origin) tier. These expose
@@ -667,6 +668,9 @@ async function readCapped(res: Response, limit: number): Promise<ArrayBuffer> {
 }
 
 async function doHttpFetch(plugin: InstalledPlugin, rawUrl: string, init?: PluginFetchInit) {
+  // Such a plugin isn't started in the no-third-party mode; this is the
+  // second lock in case one runs anyway (#903).
+  if (!pluginAllowed(plugin)) throw new Error('Outside requests are off: this server does not contact outside services');
   if (typeof rawUrl !== 'string') throw new Error('url must be a string');
   let url: URL;
   try { url = new URL(rawUrl); } catch { throw new Error('url must be absolute https://'); }
@@ -1169,6 +1173,8 @@ export interface WkdResult {
 }
 
 export async function doGetPublicKeyFromWKD(email: string): Promise<WkdResult> {
+  // The lookup goes to the recipient's domain: off with third-party traffic (#903).
+  if (!wkdAllowed()) return { status: 'ERROR', error: 'Key lookups at other domains are off on this server' };
   const cleanEmail = email.trim();
   const atIndex = cleanEmail.lastIndexOf('@');
   
diff --git a/lib/plugin-sandbox/loader.ts b/lib/plugin-sandbox/loader.ts
index a95b2bc47..4b38ee7ea 100644
--- a/lib/plugin-sandbox/loader.ts
+++ b/lib/plugin-sandbox/loader.ts
@@ -32,6 +32,7 @@ import { clearSectionTitle } from './section-titles';
 import { cancelPluginProxyRequests } from './proxy-client';
 import { cancelConnectFor } from './connect-dialog';
 import type { SlotName } from '../plugin-types';
+import { PLUGIN_REFUSED_ERROR, pluginAllowed } from '../outgoing-hosts';
 
 // ─── Hook-bus lookup (one flat map for name → bus) ────────────
 
@@ -222,6 +223,13 @@ export function loadSandboxedPlugin(plugin: InstalledPlugin): Promise<void> {
 }
 
 async function startSandboxedPlugin(plugin: InstalledPlugin): Promise<void> {
+  // A plugin that declares network access doesn't start while third-party
+  // traffic is off; the rest run (#903).
+  if (!pluginAllowed(plugin)) {
+    storeAccessor?.setPluginStatus(plugin.id, 'error', PLUGIN_REFUSED_ERROR);
+    console.warn(`[plugin-sandbox] "${plugin.id}" not started: it declares network access and third-party traffic is off`);
+    return;
+  }
 
   let background: ReturnType<typeof createBackgroundInstance> | null = null;
   try {
diff --git a/lib/push-relays.ts b/lib/push-relays.ts
index a6bc1f02d..a8185e4d5 100644
--- a/lib/push-relays.ts
+++ b/lib/push-relays.ts
@@ -3,6 +3,7 @@
 // registration (which carries their address label) at an arbitrary host.
 
 import type { PushRelayOption, SettingsPolicy } from '@/lib/admin/types';
+import { relayAllowed } from '@/lib/outgoing-hosts';
 
 // Hosted relay so self-hosters don't need their own VAPID + Firebase setup.
 // The built-in entry can be repointed at build time via NEXT_PUBLIC_PUSH_RELAY_URL.
@@ -34,7 +35,9 @@ export function relayDisplayLabel(option: PushRelayOption): string {
 
 /**
  * The relays a user may choose between: the built-in default first, then the
- * admin-configured list. Invalid and duplicate URLs are dropped.
+ * admin-configured list. Invalid and duplicate URLs are dropped, and so is
+ * the built-in relay while third-party traffic is off (#903): push then
+ * works only with a relay the admin runs.
  */
 export function resolvePushRelayOptions(policy: PushRelayPolicy | undefined | null): PushRelayOption[] {
   const options: PushRelayOption[] = [];
@@ -42,7 +45,7 @@ export function resolvePushRelayOptions(policy: PushRelayPolicy | undefined | nu
 
   const add = (label: string | undefined, rawUrl: string | undefined) => {
     const url = normalizeRelayUrl(rawUrl);
-    if (!isValidRelayUrl(url) || seen.has(url)) return;
+    if (!isValidRelayUrl(url) || seen.has(url) || !relayAllowed(url)) return;
     seen.add(url);
     options.push({ label: relayDisplayLabel({ label: label ?? '', url }), url });
   };
@@ -56,10 +59,17 @@ export function resolvePushRelayOptions(policy: PushRelayPolicy | undefined | nu
   return options;
 }
 
-/** The relay used when the user has not picked one (or their pick is gone). */
+/**
+ * The relay used when the user has not picked one (or their pick is gone).
+ * Empty when none may be used: third-party traffic is off and the admin
+ * configured no relay of their own.
+ */
 export function resolveDefaultRelayUrl(policy: PushRelayPolicy | undefined | null): string {
   const configured = normalizeRelayUrl(policy?.pushRelayUrl);
-  return isValidRelayUrl(configured) ? configured : normalizeRelayUrl(DEFAULT_RELAY_BASE_URL);
+  if (isValidRelayUrl(configured) && relayAllowed(configured)) return configured;
+  const builtIn = normalizeRelayUrl(DEFAULT_RELAY_BASE_URL);
+  if (relayAllowed(builtIn)) return builtIn;
+  return resolvePushRelayOptions(policy)[0]?.url ?? '';
 }
 
 /**
diff --git a/lib/telemetry/sender.ts b/lib/telemetry/sender.ts
index 1ee4de795..dec1c52c6 100644
--- a/lib/telemetry/sender.ts
+++ b/lib/telemetry/sender.ts
@@ -20,6 +20,8 @@ export async function sendOnce(opts?: { reason?: string }): Promise<{
   status?: number;
   error?: string;
 }> {
+  // effectiveConsent() is off in the no-third-party mode, so this also stops
+  // a manual "send now" before any request (#903).
   const { consent, source, state } = await effectiveConsent();
   if (consent !== 'on') return { ok: false, error: `consent ${consent} (source ${source})` };
   const endpoint = state.endpoint || DEFAULT_ENDPOINT;
diff --git a/lib/telemetry/state.ts b/lib/telemetry/state.ts
index 306b86ba9..ef211e8f6 100644
--- a/lib/telemetry/state.ts
+++ b/lib/telemetry/state.ts
@@ -5,6 +5,7 @@ import { randomUUID } from 'node:crypto';
 import { logger } from '@/lib/logger';
 import type { TelemetryStateFile, ConsentState } from './types';
 import { DEFAULT_ENDPOINT } from './types';
+import { isNoThirdPartyTrafficOn } from '@/lib/admin/third-party-mode';
 
 function getDir(): string {
   return process.env.TELEMETRY_DATA_DIR ||
@@ -83,15 +84,17 @@ export async function saveState(state: TelemetryStateFile): Promise<void> {
   await rename(tmp, statePath());
 }
 
-// Effective consent: env var wins over file. UI changes are blocked
-// when env override is active so the user knows where it's coming from.
+// Effective consent: the no-third-party mode wins over everything (#903),
+// then the env var, then the file. UI changes are blocked when either
+// override is active so the user knows where it's coming from.
 export async function effectiveConsent(): Promise<{
   consent: ConsentState;
-  source: 'env' | 'file';
+  source: 'no-third-party' | 'env' | 'file';
   state: TelemetryStateFile;
 }> {
   const envState = envOverride();
   const state = await loadState();
+  if (await isNoThirdPartyTrafficOn()) return { consent: 'off', source: 'no-third-party', state };
   if (envState) return { consent: envState, source: 'env', state };
   return { consent: state.consent, source: 'file', state };
 }
diff --git a/lib/third-party-mode.ts b/lib/third-party-mode.ts
new file mode 100644
index 000000000..b50dad448
--- /dev/null
+++ b/lib/third-party-mode.ts
@@ -0,0 +1,26 @@
+/**
+ * The "no third-party traffic" mode (#903) as the browser sees it.
+ *
+ * The server reads the switch from config (lib/admin/third-party-mode.ts) and
+ * hands it to the client through /api/config; Lite reads it from its
+ * config.json. fetchConfig() stores it here, and every browser-side switch in
+ * lib/outgoing-hosts.ts asks this module, so a component never has to thread
+ * the flag through props to stay off an outside host.
+ */
+
+let enabled = false;
+
+/** NO_THIRD_PARTY_TRAFFIC: true, 1, yes or on (any case) turn it on; anything else is off. */
+export function parseNoThirdPartyEnv(value: string | undefined | null): boolean {
+  return /^(true|1|yes|on)$/i.test((value ?? '').trim());
+}
+
+/** Called once the runtime config is known. */
+export function setNoThirdPartyTraffic(on: boolean): void {
+  enabled = on;
+}
+
+/** Whether the browser must stay off every built-in outside host. */
+export function noThirdPartyTraffic(): boolean {
+  return enabled;
+}
diff --git a/lib/version-check/sender.ts b/lib/version-check/sender.ts
index 49d3fe767..d2b692ae4 100644
--- a/lib/version-check/sender.ts
+++ b/lib/version-check/sender.ts
@@ -4,6 +4,7 @@ import {
 } from './state';
 import { fetchStatus } from './fetcher';
 import type { UpdateStatus } from './types';
+import { isNoThirdPartyTrafficOn } from '@/lib/admin/third-party-mode';
 
 const HOUR_MS = 60 * 60 * 1000;
 const JITTER_MS = 5 * 60 * 1000;       // ± 5 min, keeps containers from syncing
@@ -12,6 +13,9 @@ const FAILURE_BACKOFF_MS = 15 * 60 * 1000; // after a failed fetch, retry in 15
 
 let currentTimer: NodeJS.Timeout | null = null;
 
+/** What checkOnce reports while NO_THIRD_PARTY_TRAFFIC is on. */
+export const NO_THIRD_PARTY_ERROR = 'off: no third-party traffic';
+
 function jitteredDelay(base: number): number {
   const j = (Math.random() * 2 - 1) * JITTER_MS;
   return Math.max(60_000, base + j);
@@ -23,6 +27,7 @@ export async function checkOnce(opts?: { reason?: string }): Promise<{
   error?: string;
 }> {
   if (disabledByEnv()) return { ok: false, error: 'disabled by env' };
+  if (await isNoThirdPartyTrafficOn()) return { ok: false, error: NO_THIRD_PARTY_ERROR };
 
   const state = await loadState();
   const endpoint = effectiveEndpoint(state);
@@ -75,6 +80,10 @@ export async function startScheduler(): Promise<void> {
     logger.info('version-check: scheduler not started (disabled by env)');
     return;
   }
+  if (await isNoThirdPartyTrafficOn()) {
+    logger.info('version-check: scheduler not started (no third-party traffic)');
+    return;
+  }
   const state = await loadState();
   if (!effectiveEndpoint(state)) {
     logger.info('version-check: scheduler not started (no endpoint)');
diff --git a/lib/web-push.ts b/lib/web-push.ts
index 47ca9fddb..37719da8a 100644
--- a/lib/web-push.ts
+++ b/lib/web-push.ts
@@ -8,6 +8,8 @@ import type { IJMAPClient } from '@/lib/jmap/client-interface';
 import type { AccountInfo, EmailPushConfig, Mailbox } from '@/lib/jmap/types';
 import { isJmapSetError, jmapErrorType } from '@/lib/jmap/jmap-error';
 import { DEFAULT_RELAY_BASE_URL } from '@/lib/push-relays';
+import { relayAllowed } from '@/lib/outgoing-hosts';
+import { noThirdPartyTraffic } from '@/lib/third-party-mode';
 import { IS_LITE } from '@/lib/lite';
 import {
   EMPTY_NOTIFICATION_MUTES,
@@ -433,7 +435,9 @@ export function isWebPushSupported(): boolean {
   );
 }
 
+/** Every relay request is built here, so the built-in relay can't be reached in the no-third-party mode (#903). */
 function buildRelayUrl(base: string, suffix: string): string {
+  if (!relayAllowed(base)) throw new Error('The built-in push relay is off: this server does not contact outside services');
   return base.replace(/\/+$/, '') + suffix;
 }
 
@@ -893,7 +897,7 @@ export async function disableWebPush(params: DisableWebPushParams): Promise<void
   // Enabling again starts from the full emailPush map.
   localStorage.removeItem(emailPushRefusedKey(accountId));
 
-  if (deviceClientId && relayBaseUrl) {
+  if (deviceClientId && relayBaseUrl && relayAllowed(relayBaseUrl)) {
     await fetch(
       buildRelayUrl(relayBaseUrl, `/api/push/register/${encodeURIComponent(deviceClientId)}`),
       { method: 'DELETE' },
@@ -980,7 +984,7 @@ export async function revokePushDevice(params: {
   }
 
   await params.client.destroyPushSubscription(params.device.id);
-  if (relayBaseUrl && params.device.deviceClientId) {
+  if (relayBaseUrl && relayAllowed(relayBaseUrl) && params.device.deviceClientId) {
     await fetch(
       buildRelayUrl(
         relayBaseUrl,
@@ -1071,6 +1075,14 @@ async function runResync(accountId: string, params: ResyncWebPushParams): Promis
     // isWebPushEnabled (it requires a live browser subscription) would do.
     if (!isWebPushSupported() || Notification.permission !== 'granted'
       || !localStorage.getItem(subscriptionIdKey(accountId))) return false;
+    // Registered with the built-in relay before third-party traffic was
+    // turned off, and no relay of the organisation's own to move to: drop the
+    // server's PushSubscription so the mail server stops pinging that relay
+    // (#903). disableWebPush skips the relay itself.
+    if (!params.relayBaseUrl && noThirdPartyTraffic()) {
+      await disableWebPush({ client: params.client });
+      return false;
+    }
     if (!params.force && !(await resyncDue(accountId))) return false;
     await enableWebPush({
       client: params.client,
diff --git a/locales/de/common.json b/locales/de/common.json
index ffdd16235..65b21f168 100644
--- a/locales/de/common.json
+++ b/locales/de/common.json
@@ -981,6 +981,12 @@
     "rule_folder_failed": "Der Ordner konnte nicht erstellt werden",
     "vacation_filters_save_failed": "Abwesenheits-Einstellungen gespeichert, Weiterleitung und Antwortempfänger konnten aber nicht gespeichert werden"
   },
+  "third_party": {
+    "push_needs_relay": "Push-Benachrichtigungen brauchen ein Push-Relay Ihrer Organisation, und es ist keines eingerichtet. Neue E-Mails werden weiterhin angezeigt, solange die App geöffnet ist.",
+    "relay_desc": "Es werden nur die eigenen Relays Ihrer Organisation angeboten, weil dieser Server keine externen Dienste kontaktiert.",
+    "relay_none": "Keines eingerichtet",
+    "plugin_refused": "Nicht gestartet: Dieses Plugin verbindet sich mit externen Diensten, die Ihre Organisation abgeschaltet hat."
+  },
   "date": {
     "today": "Heute",
     "yesterday": "Gestern",
diff --git a/locales/en/common.json b/locales/en/common.json
index 95ec87fa3..a72f8d778 100644
--- a/locales/en/common.json
+++ b/locales/en/common.json
@@ -981,6 +981,12 @@
     "rule_folder_failed": "Could not create the folder",
     "vacation_filters_save_failed": "Out of office settings saved, but the forwarding and reply recipients could not be saved"
   },
+  "third_party": {
+    "push_needs_relay": "Push notifications need a push relay run by your organisation, and none is set up. New mail still shows while the app is open.",
+    "relay_desc": "Only your organisation's own relays are offered, because this server doesn't contact outside services.",
+    "relay_none": "None set up",
+    "plugin_refused": "Not started: this plugin connects to outside services, which your organisation has turned off."
+  },
   "date": {
     "today": "Today",
     "yesterday": "Yesterday",
diff --git a/scripts/lite/build.mjs b/scripts/lite/build.mjs
index fa1a22689..cfa36568e 100644
--- a/scripts/lite/build.mjs
+++ b/scripts/lite/build.mjs
@@ -10,6 +10,7 @@
 //
 // Inputs (env): LITE_TARGET (static | stalwart), LITE_JMAP_SERVER_URL,
 // LITE_APP_NAME, LITE_ALLOW_CUSTOM_ENDPOINT, LITE_REMEMBER_ME, LITE_DEMO_MODE,
+// LITE_NO_THIRD_PARTY_TRAFFIC,
 // LITE_LOCALES, NEXT_PUBLIC_BASE_PATH (static target only),
 // NEXT_PUBLIC_DEFAULT_LOCALE, GIT_COMMIT. Flags: --target=stalwart,
 // --skip-prepare (tree already pruned), --dry-run (print the plan only).
diff --git a/scripts/lite/lib.mjs b/scripts/lite/lib.mjs
index 08a811f54..0d35a9fce 100644
--- a/scripts/lite/lib.mjs
+++ b/scripts/lite/lib.mjs
@@ -168,6 +168,8 @@ export function buildLiteConfig(env = {}, { target = "static" } = {}) {
       : {}),
     rememberMeEnabled: parseBool(env.LITE_REMEMBER_ME, true),
     demoMode: parseBool(env.LITE_DEMO_MODE, false),
+    // Written out so deployers find the switch (#903; lib/outgoing-hosts.ts).
+    noThirdPartyTraffic: parseBool(env.LITE_NO_THIRD_PARTY_TRAFFIC, false),
     loginShowTotp: true,
     loginShowVersion: true,
   };
@@ -661,6 +663,8 @@ ${demoMode ? "Demo mode is ON: the login page offers a built-in demo account and
      while \`jmapServerUrl\` is empty.
    - \`rememberMeEnabled\`: \`false\` never remembers a sign-in (sessions then end with the tab).
      Sign-ins are otherwise remembered; \`loginShowRememberMe: true\` lets people choose.
+   - \`noThirdPartyTraffic\`: \`true\` keeps the browser off every built-in outside host
+     (no Google Maps links on event addresses; see the third-party traffic docs).
    Optional keys: \`demoMode\`, \`jmapServers\`, \`jmapServerAutoPickByDomain\`, the login logo/company/link keys and \`loginShow*\` toggles (same names as the Docker env vars, camelCased).
 3. Allow the browser to talk to the mail server (CORS). In Stalwart, turn on
    the \`usePermissiveCors\` setting of the \`Http\` object: in the web admin