diff --git a/lib/__tests__/outgoing-hosts-mode.test.ts b/lib/__tests__/outgoing-hosts-mode.test.ts
new file mode 100644
index 000000000..be4a37094
--- /dev/null
+++ b/lib/__tests__/outgoing-hosts-mode.test.ts
@@ -0,0 +1,118 @@
+// @vitest-environment node
+import { afterAll, 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';
+import {
+  OUTGOING_HOSTS,
+  isBuiltInHost,
+  mapsLinksAllowed,
+  sampleAvatarsAllowed,
+  senderFaviconsAllowed,
+  wkdAllowed,
+} from '@/lib/outgoing-hosts';
+import { setNoThirdPartyTraffic } from '@/lib/third-party-mode';
+
+// Turns "no third-party traffic" on the way an admin does (config.json; the
+// browser gets it from /api/config) and checks that every entry of the
+// registry is then off, by asking the code that uses each host. A new entry
+// without a probe here fails, so the registry can't list a host whose
+// switch nobody checked (#903).
+
+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 }));
+
+import { configManager } from '@/lib/admin/config-manager';
+
+let dir: string;
+const fetchMock = vi.fn(async (_url: unknown, _init?: unknown) => Response.json({ latest: '1.18.0' }));
+const fetchedHosts = () => fetchMock.mock.calls.map(([url]) => new URL(String(url)).hostname);
+
+async function setMode(on: boolean) {
+  writeFileSync(path.join(dir, 'config.json'), JSON.stringify(on ? { noThirdPartyTraffic: true } : {}));
+  await configManager.load();
+  // What the browser does with /api/config (hooks/use-config.ts).
+  const { GET } = await import('@/app/api/config/route');
+  const config = await (await GET(new NextRequest('https://webmail.example/api/config'))).json();
+  setNoThirdPartyTraffic(config.noThirdPartyTraffic === true);
+}
+
+/** Whether the code behind each registry entry would reach its host right now. */
+const probes: Record<string, () => Promise<boolean>> = {
+  telemetry: async () => {
+    const { effectiveConsent, loadState, saveState } = await import('@/lib/telemetry');
+    await saveState({ ...(await loadState()), consent: 'on' });
+    return (await effectiveConsent()).consent === 'on';
+  },
+  'version-check': async () => {
+    const { checkOnce } = await import('@/lib/version-check');
+    await checkOnce();
+    return fetchedHosts().includes('version.telemetry.bulwarkmail.org');
+  },
+  'push-relay': async () => {
+    const { resolvePushRelayOptions, resolveDefaultRelayUrl } = await import('@/lib/push-relays');
+    return resolvePushRelayOptions({}).some((o) => isBuiltInHost(o.url)) || isBuiltInHost(resolveDefaultRelayUrl({}));
+  },
+  'extension-directory': async () => {
+    const { directoryOff } = await import('@/lib/admin/marketplace-directory');
+    return !(await directoryOff());
+  },
+  connector: async () => {
+    const { GET } = await import('@/app/api/connector/capabilities/route');
+    return (await GET()).status === 200;
+  },
+  favicons: async () => {
+    const { GET } = await import('@/app/api/favicon/route');
+    await GET(new NextRequest(`https://webmail.example/api/favicon?domain=probe${Date.now()}.org`));
+    return fetchedHosts().includes('icons.duckduckgo.com') || senderFaviconsAllowed();
+  },
+  translate: async () => {
+    const { POST } = await import('@/app/api/translate/route');
+    await POST(new NextRequest('https://webmail.example/api/translate', {
+      method: 'POST', body: JSON.stringify({ text: 'Hallo Welt', target: 'en', source: 'de' }),
+    }));
+    return fetchedHosts().includes('api.mymemory.translated.net');
+  },
+  maps: async () => mapsLinksAllowed(),
+  wkd: async () => wkdAllowed(),
+  'dev-avatars': async () => sampleAvatarsAllowed(),
+};
+
+beforeAll(() => {
+  dir = mkdtempSync(path.join(tmpdir(), 'bulwark-3p-mode-'));
+  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');
+  delete process.env.NO_THIRD_PARTY_TRAFFIC;
+});
+afterAll(() => {
+  setNoThirdPartyTraffic(false);
+  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);
+});
+
+describe('no third-party traffic mode', () => {
+  it('has a probe for every registry entry', () => {
+    expect(Object.keys(probes).sort()).toEqual(OUTGOING_HOSTS.map((h) => h.id).sort());
+  });
+
+  it.each(OUTGOING_HOSTS.map((h) => [h.id, h] as const))('%s is reachable with the mode off', async (id) => {
+    await setMode(false);
+    expect(await probes[id]()).toBe(true);
+  });
+
+  it.each(OUTGOING_HOSTS.map((h) => [h.id, h] as const))('%s is off with the mode on', async (id) => {
+    await setMode(true);
+    expect(await probes[id]()).toBe(false);
+  });
+});
diff --git a/lib/__tests__/outgoing-hosts.test.ts b/lib/__tests__/outgoing-hosts.test.ts
new file mode 100644
index 000000000..15342981e
--- /dev/null
+++ b/lib/__tests__/outgoing-hosts.test.ts
@@ -0,0 +1,110 @@
+// @vitest-environment node
+import { describe, expect, it } from 'vitest';
+import { readdirSync, readFileSync, statSync } from 'node:fs';
+import path from 'node:path';
+import {
+  NON_REQUEST_HOSTS,
+  NON_REQUEST_PATHS,
+  OUTGOING_HOSTS,
+  isPlaceholderHost,
+  registeredLiterals,
+} from '@/lib/outgoing-hosts';
+
+// Every http(s) host literal in the shipped source must be a built-in
+// outside host in lib/outgoing-hosts.ts (with a switch that turns it off) or
+// a known non-request (docs link, namespace, sample text). A new outside
+// host fails here until someone decides which it is (#903).
+
+const ROOT = path.resolve(__dirname, '..', '..');
+const DIRS = ['app', 'lib', 'stores', 'hooks', 'components', 'contexts'];
+const FILES = ['public/sw.js', 'proxy.ts', 'instrumentation.ts', 'instrumentation.node.ts'];
+const EXTENSIONS = /\.(ts|tsx|js|jsx|mjs|cjs)$/;
+const HOST_LITERAL = /https?:\/\/([A-Za-z0-9.*_-]+)/g;
+
+function isTestPath(rel: string): boolean {
+  return /(^|\/)(__tests__|__mocks__|node_modules)\//.test(rel) || /\.(test|spec)\.[a-z]+$/.test(rel);
+}
+
+function walk(rel: string, out: string[]) {
+  const abs = path.join(ROOT, rel);
+  for (const name of readdirSync(abs)) {
+    const child = `${rel}/${name}`;
+    if (isTestPath(`${child}/`) || name === 'node_modules') continue;
+    if (statSync(path.join(abs, name)).isDirectory()) walk(child, out);
+    else if (EXTENSIONS.test(name) && !isTestPath(child)) out.push(child);
+  }
+}
+
+export function sourceFiles(): string[] {
+  const out: string[] = [];
+  for (const dir of DIRS) walk(dir, out);
+  for (const file of FILES) {
+    try { statSync(path.join(ROOT, file)); out.push(file); } catch { /* not in this checkout */ }
+  }
+  return out.filter((file) => !NON_REQUEST_PATHS.some((prefix) => file.startsWith(prefix)));
+}
+
+/** Host literals in a source text, as written (lower case, port dropped). */
+export function hostLiterals(text: string): string[] {
+  return [...text.matchAll(HOST_LITERAL)].map((m) => m[1].toLowerCase());
+}
+
+/** Why a host literal in `file` is fine, or null when it isn't accounted for. */
+export function classify(host: string, file: string, registered = registeredLiterals()): string | null {
+  if (registered.has(host)) return 'registry';
+  const trimmed = host.replace(/\.+$/, '');
+  if (registered.has(trimmed)) return 'registry';
+  const allowed = NON_REQUEST_HOSTS.find((entry) => entry.host === trimmed);
+  if (allowed && allowed.files.includes(file)) return 'non-request';
+  if (isPlaceholderHost(host)) return 'placeholder';
+  return null;
+}
+
+describe('outgoing host literals', () => {
+  const files = sourceFiles();
+
+  it('scans the shipped source, service worker included, and leaves the tests out', () => {
+    expect(files).toContain('public/sw.js');
+    expect(files).toContain('app/api/favicon/route.ts');
+    expect(files.some((f) => f.includes('__tests__'))).toBe(false);
+    expect(files.some((f) => f.startsWith('lib/demo/fixtures/'))).toBe(false);
+    expect(files.length).toBeGreaterThan(500);
+  });
+
+  it('finds no host that is neither registered nor a known non-request', () => {
+    const registered = registeredLiterals();
+    const unknown: string[] = [];
+    for (const file of files) {
+      const text = readFileSync(path.join(ROOT, file), 'utf8');
+      for (const host of hostLiterals(text)) {
+        if (!classify(host, file, registered)) unknown.push(`${file}: ${host}`);
+      }
+    }
+    expect(unknown, 'Add the host to OUTGOING_HOSTS with a switch, or to NON_REQUEST_HOSTS if Bulwark never connects to it').toEqual([]);
+  });
+
+  it('lists only hosts that are still in the source', () => {
+    const all = files.map((file) => ({ file, text: readFileSync(path.join(ROOT, file), 'utf8') }));
+    const literals = new Set(all.flatMap(({ text }) => hostLiterals(text)));
+    for (const entry of OUTGOING_HOSTS) {
+      for (const literal of entry.literals ?? [entry.host]) {
+        expect(literals.has(literal), `${entry.id}: ${literal}`).toBe(true);
+      }
+    }
+    for (const entry of NON_REQUEST_HOSTS) {
+      for (const file of entry.files) {
+        const text = all.find((f) => f.file === file)?.text ?? '';
+        expect(hostLiterals(text).some((h) => h.replace(/\.+$/, '') === entry.host), `${entry.host} in ${file}`).toBe(true);
+      }
+    }
+  });
+
+  it('classifies the way the registry says', () => {
+    expect(classify('icons.duckduckgo.com', 'app/api/favicon/route.ts')).toBe('registry');
+    expect(classify('openpgpkey.', 'lib/plugin-sandbox/host-api.ts')).toBe('registry');
+    expect(classify('github.com', 'components/settings/about-data-settings.tsx')).toBe('non-request');
+    expect(classify('github.com', 'lib/some-new-file.ts')).toBeNull();
+    expect(classify('mail.example.com', 'anywhere.ts')).toBe('placeholder');
+    expect(classify('cdn.unpkg.com', 'lib/x.ts')).toBeNull();
+  });
+});