diff --git a/src/components/EmailBodyView.tsx b/src/components/EmailBodyView.tsx
index 7acac418..f63e57f6 100644
--- a/src/components/EmailBodyView.tsx
+++ b/src/components/EmailBodyView.tsx
@@ -20,6 +20,7 @@ import { buildMailTextSizeScript, mailTextFactor } from '../lib/mail-reading';
 import { buildPauseAnimatedImagesScript, withReducedMotion } from '../lib/mail-motion';
 import { useHasContacts } from '../lib/capabilities';
 import { useSettingsStore } from '../stores/settings-store';
+import { useNoThirdPartyTraffic } from '../stores/policy-store';
 import { useContactsStore } from '../stores/contacts-store';
 import { useLocaleStore } from '../stores/locale-store';
 import { spacing, typography, type ThemePalette } from '../theme/tokens';
@@ -437,6 +438,9 @@ export default function EmailBodyView({
   const t = useLocaleStore((s) => s.t);
   const navigation = useNavigation<Nav>();
   const externalContentPolicy = useSettingsStore((s) => s.externalContentPolicy);
+  // No third-party traffic (webmail #903): remote content never loads,
+  // whatever the policy or trust says, and nothing offers to load it.
+  const remoteBlockedByOrg = useNoThirdPartyTraffic();
   const isSenderTrusted = useSettingsStore((s) => s.isSenderTrusted);
   const addTrustedSender = useSettingsStore((s) => s.addTrustedSender);
   const trustedSendersAddressBook = useSettingsStore((s) => s.trustedSendersAddressBook);
@@ -523,10 +527,10 @@ export default function EmailBodyView({
   // trusted, regardless of what the detection pass finds - the pass only
   // drives the banner and the placeholder swap.
   const shouldBlock =
-    !!rawHtml &&
-    !trusted &&
-    !allowOnce &&
-    externalContentPolicy !== 'allow';
+    !!rawHtml && (
+      remoteBlockedByOrg ||
+      (!trusted && !allowOnce && externalContentPolicy !== 'allow')
+    );
 
   // The WebView's height: what the page reported, else the height this body
   // had the last time it was shown, else (see `fill`) an estimate. Never a
@@ -736,9 +740,11 @@ export default function EmailBodyView({
       {showBanner && (
         <View style={styles.banner}>
           <Text style={styles.bannerText}>
-            {t('email_viewer.external_content_warning', 'Images and external content have been blocked')}
+            {remoteBlockedByOrg
+              ? t('third_party.remote_content_blocked', "Your organisation blocks content from other servers, so remote pictures in this message aren't shown.")
+              : t('email_viewer.external_content_warning', 'Images and external content have been blocked')}
           </Text>
-          <View style={styles.bannerActions}>
+          {!remoteBlockedByOrg && <View style={styles.bannerActions}>
             {externalContentPolicy === 'ask' && (
               <Pressable style={styles.bannerButton} onPress={onLoadImages} hitSlop={8}>
                 <ImageIcon size={14} color={c.textSecondary} />
@@ -751,7 +757,7 @@ export default function EmailBodyView({
                 <Text style={styles.bannerButtonText}>{t('email_viewer.trust_sender', 'Always trust this sender')}</Text>
               </Pressable>
             ) : null}
-          </View>
+          </View>}
         </View>
       )}
       {showTruncated && (
diff --git a/src/components/__tests__/custom-from-third-party-wiring.test.ts b/src/components/__tests__/custom-from-third-party-wiring.test.ts
new file mode 100644
index 00000000..7d648ed3
--- /dev/null
+++ b/src/components/__tests__/custom-from-third-party-wiring.test.ts
@@ -0,0 +1,55 @@
+import { describe, expect, it } from 'vitest';
+import fs from 'node:fs';
+import path from 'node:path';
+import { webmailLinksFromConfig, helpLinks } from '../../lib/org-policy';
+
+// The organisation's customFromEnabled gate (webmail composing-settings.tsx,
+// email-composer.tsx) and the webmail's "no third-party traffic" mode (#903,
+// hooks/use-third-party-mode.ts, reading-settings.tsx, email-viewer.tsx). No
+// React Native renderer in the test env, so this checks the wiring.
+
+const read = (rel: string) => fs.readFileSync(path.resolve(__dirname, '..', '..', rel), 'utf8');
+
+describe('customFromEnabled', () => {
+  it('shows Received Address Matching only when custom From is allowed', () => {
+    const src = read('components/settings/ComposingSettings.tsx');
+    expect(src).toMatch(/const customFromEnabled = usePolicyFeature\('customFromEnabled'\);/);
+    expect(src).toMatch(/\{autoSelectReplyIdentity && customFromEnabled && \(/);
+  });
+
+  it('matches only exact identities in the composer when custom From is off', () => {
+    expect(read('screens/ComposeScreen.tsx')).toMatch(/matchMode: customFromEnabled \? replyIdentityMatch : 'exact',/);
+  });
+});
+
+describe('no third-party traffic', () => {
+  it('reads the mode from the webmail config, as a boolean or an env-style string', () => {
+    expect(webmailLinksFromConfig({ noThirdPartyTraffic: true }).noThirdPartyTraffic).toBe(true);
+    expect(webmailLinksFromConfig({ noThirdPartyTraffic: 'Yes' }).noThirdPartyTraffic).toBe(true);
+    expect(webmailLinksFromConfig({ noThirdPartyTraffic: false }).noThirdPartyTraffic).toBeUndefined();
+    expect(webmailLinksFromConfig({ noThirdPartyTraffic: 'no' }).noThirdPartyTraffic).toBeUndefined();
+    // Not a help link.
+    expect(helpLinks(webmailLinksFromConfig({ noThirdPartyTraffic: true }))).toEqual([]);
+  });
+
+  it('shows External Content as the organisation\'s, set to block', () => {
+    const src = read('components/settings/ContentSendersSettings.tsx');
+    expect(src).toMatch(/const remoteBlockedByOrg = useNoThirdPartyTraffic\(\);/);
+    expect(src).toMatch(/locked=\{externalContentLocked \|\| remoteBlockedByOrg\}/);
+    expect(src).toMatch(/value=\{remoteBlockedByOrg \? 'block' : externalContentPolicy\}/);
+  });
+
+  it('never loads remote content in the reader and offers nothing to load it', () => {
+    const src = read('components/EmailBodyView.tsx');
+    expect(src).toMatch(/!!rawHtml && \(\s+remoteBlockedByOrg \|\|\s+\(!trusted && !allowOnce && externalContentPolicy !== 'allow'\)\s+\)/);
+    expect(src).toMatch(/\{!remoteBlockedByOrg && <View style=\{styles\.bannerActions\}>/);
+    expect(src).toMatch(/t\('third_party\.remote_content_blocked'/);
+  });
+
+  it('keeps remote images blocked in booking cards and quoted originals', () => {
+    expect(read('components/email/structured-mail/StructuredMailBanner.tsx')).toMatch(/\(\) => !remoteBlockedByOrg && \(policy === 'allow' \|\|/);
+    const compose = read('screens/ComposeScreen.tsx');
+    expect(compose).toMatch(/externalContentPolicy: remoteBlockedByOrg \? 'block' : settings\.externalContentPolicy,/);
+    expect(compose).toMatch(/senderTrusted: !remoteBlockedByOrg && !!sender/);
+  });
+});
diff --git a/src/components/email/structured-mail/StructuredMailBanner.tsx b/src/components/email/structured-mail/StructuredMailBanner.tsx
index 136bc369..aa6ca805 100644
--- a/src/components/email/structured-mail/StructuredMailBanner.tsx
+++ b/src/components/email/structured-mail/StructuredMailBanner.tsx
@@ -8,6 +8,7 @@ import { isSenderContentTrusted, isTrustedSendersSyncOn } from '../../../lib/tru
 import { useHasContacts } from '../../../lib/capabilities';
 import { useContactsStore } from '../../../stores/contacts-store';
 import { useSettingsStore } from '../../../stores/settings-store';
+import { useNoThirdPartyTraffic } from '../../../stores/policy-store';
 import { BookingCalendarActions } from './booking-calendar-actions';
 import { StructuredMailCards } from './structured-mail-cards';
 import { useStructuredMail } from './use-structured-mail';
@@ -28,6 +29,8 @@ interface Props {
 export function StructuredMailBanner({ email, jmapAccountId }: Props) {
   const cards = useStructuredMail(email, jmapAccountId);
   const policy = useSettingsStore((s) => s.externalContentPolicy);
+  // No third-party traffic (webmail #903): remote images never load.
+  const remoteBlockedByOrg = useNoThirdPartyTraffic();
   const trustedSenders = useSettingsStore((s) => s.trustedSenders);
   const isSenderTrusted = useSettingsStore((s) => s.isSenderTrusted);
   const trustedSendersAddressBook = useSettingsStore((s) => s.trustedSendersAddressBook);
@@ -35,13 +38,13 @@ export function StructuredMailBanner({ email, jmapAccountId }: Props) {
   const hasContacts = useHasContacts();
   const sender = email.from?.[0]?.email;
   const allowRemoteImages = React.useMemo(
-    () => policy === 'allow' || isSenderContentTrusted(sender, {
+    () => !remoteBlockedByOrg && (policy === 'allow' || isSenderContentTrusted(sender, {
       isLocallyTrusted: isSenderTrusted,
       syncEnabled: isTrustedSendersSyncOn(trustedSendersAddressBook, hasContacts),
       trustedBookEmails,
-    }),
+    })),
     // trustedSenders changes what isSenderTrusted answers.
-    [policy, sender, isSenderTrusted, trustedSenders, trustedSendersAddressBook, hasContacts, trustedBookEmails], // eslint-disable-line react-hooks/exhaustive-deps
+    [remoteBlockedByOrg, policy, sender, isSenderTrusted, trustedSenders, trustedSendersAddressBook, hasContacts, trustedBookEmails], // eslint-disable-line react-hooks/exhaustive-deps
   );
   const calendarActions = React.useCallback(
     (booking: Booking) => <BookingCalendarActions booking={booking} email={email} />,
diff --git a/src/components/settings/ComposingSettings.tsx b/src/components/settings/ComposingSettings.tsx
index dd047214..75f9423e 100644
--- a/src/components/settings/ComposingSettings.tsx
+++ b/src/components/settings/ComposingSettings.tsx
@@ -7,7 +7,7 @@ import { spacing, radius, typography, type ThemePalette } from '../../theme/toke
 import { useColors } from '../../theme/colors';
 import { useSettingsStore, type MentionAddsRecipientTo, type ReplyIdentityMatch, type SignaturePosition } from '../../stores/settings-store';
 import { useLocaleStore } from '../../stores/locale-store';
-import { useSettingLocked } from '../../stores/policy-store';
+import { usePolicyFeature, useSettingLocked } from '../../stores/policy-store';
 import { SUPPORTED_SUB_ADDRESS_DELIMITERS } from '../../lib/sub-addressing';
 import { RADIO_GROUP_A11Y, radioOptionA11y } from '../radio-a11y';
 
@@ -29,6 +29,9 @@ export function ComposingSettings() {
   const autoSelectReplyIdentityLocked = useSettingLocked('autoSelectReplyIdentity');
   const replyIdentityMatchLocked = useSettingLocked('replyIdentityMatch');
   const plainTextModeLocked = useSettingLocked('plainTextMode');
+  // Catch-all matching only rewrites From, which the organisation can turn
+  // off (webmail composing-settings.tsx customFromEnabled).
+  const customFromEnabled = usePolicyFeature('customFromEnabled');
   const signaturePosition = useSettingsStore((s) => s.signaturePosition);
   const signatureSeparatorEnabled = useSettingsStore((s) => s.signatureSeparatorEnabled);
   const requestReadReceiptDefault = useSettingsStore((s) => s.requestReadReceiptDefault);
@@ -135,7 +138,7 @@ export function ComposingSettings() {
         <ToggleSwitch checked={autoSelectReplyIdentity} onChange={setAutoSelectReplyIdentity} />
       </SettingItem>
 
-      {autoSelectReplyIdentity && (
+      {autoSelectReplyIdentity && customFromEnabled && (
         <View style={styles.subBlock}>
           <Text style={styles.subLabel}>{t('settings.email_behavior.reply_identity_match.label', 'Received Address Matching')}</Text>
           <Text style={styles.subDescription}>
diff --git a/src/components/settings/ContentSendersSettings.tsx b/src/components/settings/ContentSendersSettings.tsx
index 95d0e518..e1f1a0aa 100644
--- a/src/components/settings/ContentSendersSettings.tsx
+++ b/src/components/settings/ContentSendersSettings.tsx
@@ -13,7 +13,7 @@ import { useHasContacts } from '../../lib/capabilities';
 import { isTrustedSendersSyncOn } from '../../lib/trusted-senders';
 import { removeTrustedSenderWithUndo } from '../../lib/settings-undo';
 import { useLocaleStore } from '../../stores/locale-store';
-import { usePolicyFeature, useSettingHidden, useSettingLocked } from '../../stores/policy-store';
+import { useNoThirdPartyTraffic, usePolicyFeature, useSettingHidden, useSettingLocked } from '../../stores/policy-store';
 import { MODAL_ORIENTATIONS } from '../../lib/modal-orientations';
 import { useSheetBottomInset } from '../../lib/sheet-bottom-inset';
 import { ToastHost } from '../ToastHost';
@@ -38,6 +38,9 @@ export function ContentSendersSettings() {
   const externalContentHidden = useSettingHidden('externalContentPolicy');
   const externalContentLocked = useSettingLocked('externalContentPolicy');
   const showExternalContent = externalContentAllowed && !externalContentHidden;
+  // No third-party traffic (webmail #903) blocks remote content outright:
+  // the setting shows as the organisation's, set to block.
+  const remoteBlockedByOrg = useNoThirdPartyTraffic();
   const emailAlwaysLightMode = useSettingsStore((s) => s.emailAlwaysLightMode);
   const setEmailAlwaysLightMode = useSettingsStore((s) => s.setEmailAlwaysLightMode);
   const trustedSenders = useSettingsStore((s) => s.trustedSenders);
@@ -145,10 +148,10 @@ export function ContentSendersSettings() {
           <SettingItem
             label={t('settings.email_behavior.external_content.label', 'External Content')}
             description={t('settings.email_behavior.external_content.description', 'How to handle images and external content')}
-            locked={externalContentLocked}
+            locked={externalContentLocked || remoteBlockedByOrg}
           >
             <RadioGroup
-              value={externalContentPolicy}
+              value={remoteBlockedByOrg ? 'block' : externalContentPolicy}
               onChange={(v) => setExternalContentPolicy(v as ExternalContentPolicy)}
               options={[
                 { value: 'ask', label: t('settings.email_behavior.external_content.ask_mobile', 'Ask') },
diff --git a/src/components/settings/__tests__/org-policy-settings.test.ts b/src/components/settings/__tests__/org-policy-settings.test.ts
index a5bd4be8..7e0a4e6a 100644
--- a/src/components/settings/__tests__/org-policy-settings.test.ts
+++ b/src/components/settings/__tests__/org-policy-settings.test.ts
@@ -40,7 +40,7 @@ describe('organisation policy on the settings pages', () => {
     expect(page).toContain("const externalContentAllowed = usePolicyFeature('externalContentEnabled');");
     expect(page).toContain("const externalContentHidden = useSettingHidden('externalContentPolicy');");
     expect(page).toContain('const showExternalContent = externalContentAllowed && !externalContentHidden;');
-    expect(page).toMatch(/\{showExternalContent && \(\s*<SettingItem\s+label=\{t\('settings\.email_behavior\.external_content\.label'[\s\S]*?locked=\{externalContentLocked\}/);
+    expect(page).toMatch(/\{showExternalContent && \(\s*<SettingItem\s+label=\{t\('settings\.email_behavior\.external_content\.label'[\s\S]*?locked=\{externalContentLocked(?: \|\| remoteBlockedByOrg)?\}/);
     expect(page).toContain("const externalContentLocked = useSettingLocked('externalContentPolicy');");
   });
 
diff --git a/src/lib/org-policy.ts b/src/lib/org-policy.ts
index 3d167746..1278549c 100644
--- a/src/lib/org-policy.ts
+++ b/src/lib/org-policy.ts
@@ -480,10 +480,18 @@ export interface WebmailLinks {
   signLanguageUrl?: string;
   /** The config's company name (`loginCompanyName`), for the organisation signature's {company}. */
   companyName?: string;
+  /**
+   * The webmail's "no third-party traffic" mode (#903, config
+   * `noThirdPartyTraffic`): remote content in mail never loads.
+   */
+  noThirdPartyTraffic?: boolean;
 }
 
 /** The accessibility and help links, in the webmail's fixed order (lib/help-links.ts). */
-export const HELP_LINK_ORDER: ReadonlyArray<[HelpLinkId, keyof WebmailLinks]> = [
+/** The keys of WebmailLinks that hold a link. */
+export type WebmailLinkKey = Exclude<keyof WebmailLinks, 'companyName' | 'noThirdPartyTraffic'>;
+
+export const HELP_LINK_ORDER: ReadonlyArray<[HelpLinkId, WebmailLinkKey]> = [
   ['statement', 'accessibilityStatementUrl'],
   ['feedback', 'accessibilityFeedbackUrl'],
   ['help', 'helpUrl'],
@@ -506,6 +514,11 @@ export function webmailLinksFromConfig(raw: unknown): WebmailLinks {
   if (typeof raw.loginCompanyName === 'string' && raw.loginCompanyName.trim()) {
     out.companyName = raw.loginCompanyName.trim().slice(0, 200);
   }
+  // A boolean, or a string as in env (webmail lib/third-party-mode.ts parseNoThirdPartyValue).
+  const noThirdParty = raw.noThirdPartyTraffic;
+  if (noThirdParty === true || (typeof noThirdParty === 'string' && /^(true|1|yes|on)$/i.test(noThirdParty.trim()))) {
+    out.noThirdPartyTraffic = true;
+  }
   return out;
 }
 
diff --git a/src/screens/ComposeScreen.tsx b/src/screens/ComposeScreen.tsx
index 4f731f37..0dd64dc1 100644
--- a/src/screens/ComposeScreen.tsx
+++ b/src/screens/ComposeScreen.tsx
@@ -35,7 +35,7 @@ import { useSheetBottomInset } from '../lib/sheet-bottom-inset';
 import { createAfterDismissQueue } from '../lib/after-modal-dismiss';
 import { useLocaleStore } from '../stores/locale-store';
 import { useSettingsStore } from '../stores/settings-store';
-import { usePolicyFeature, usePolicyStore } from '../stores/policy-store';
+import { useNoThirdPartyTraffic, usePolicyFeature, usePolicyStore } from '../stores/policy-store';
 import { effectiveSignatureIdentity, finalizeOrgOutgoing, orgSignatureSetup, htmlCarriesDisclaimer, textCarriesDisclaimer } from '../lib/org-signature';
 import { useHasContacts } from '../lib/capabilities';
 import { isTrustedSendersSyncOn } from '../lib/trusted-senders';
@@ -416,6 +416,9 @@ export default function ComposeScreen({ route, navigation }: Props) {
 
   const autoSelectReplyIdentity = useSettingsStore((s) => s.autoSelectReplyIdentity);
   const replyIdentityMatch = useSettingsStore((s) => s.replyIdentityMatch);
+  const customFromEnabled = usePolicyFeature('customFromEnabled');
+  // No third-party traffic (webmail #903): a quoted original's remote images stay blocked.
+  const remoteBlockedByOrg = useNoThirdPartyTraffic();
   // Per-message format (#1022): seeded from the "plain text only" setting,
   // or the format a reopened draft was written in, and switchable from the
   // toolbar for just this message.
@@ -518,8 +521,8 @@ export default function ComposeScreen({ route, navigation }: Props) {
     return shouldBlockEditorRemoteImages({
       seedHtml: draft ? draft.htmlBody : replyTo?.htmlBody,
       isDraft: !!draft,
-      externalContentPolicy: settings.externalContentPolicy,
-      senderTrusted: !!sender && (
+      externalContentPolicy: remoteBlockedByOrg ? 'block' : settings.externalContentPolicy,
+      senderTrusted: !remoteBlockedByOrg && !!sender && (
         settings.isSenderTrusted(sender)
         || useContactsStore.getState().trustedSenderEmails.includes(sender)
       ),
@@ -972,7 +975,8 @@ export default function ComposeScreen({ route, navigation }: Props) {
       const resolved = resolveReplyIdentity(identities, replyTo, {
         ownEmails,
         catchAll: autoSelectReplyIdentity && mode !== 'forward',
-        matchMode: replyIdentityMatch,
+        // Without the organisation's custom From, only an exact identity (webmail email-composer.tsx).
+        matchMode: customFromEnabled ? replyIdentityMatch : 'exact',
       });
       if (resolved) {
         setSelectedIdentityId(resolved.identityId);
@@ -984,7 +988,7 @@ export default function ComposeScreen({ route, navigation }: Props) {
     }
     setSelectedIdentityId(defaultIdentity.id);
     // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [identities, autoSelectReplyIdentity, replyIdentityMatch, replyTo, draft, selectedIdentityId, preferredIdentityIds]);
+  }, [identities, autoSelectReplyIdentity, replyIdentityMatch, customFromEnabled, replyTo, draft, selectedIdentityId, preferredIdentityIds]);
 
   const primaryIdentity = React.useMemo(() => {
     if (identities.length === 0) return null;
diff --git a/src/stores/policy-store.ts b/src/stores/policy-store.ts
index 664cfa3e..50115421 100644
--- a/src/stores/policy-store.ts
+++ b/src/stores/policy-store.ts
@@ -267,6 +267,15 @@ export function useSettingLocked(key: string): boolean {
   return usePolicyStore((s) => isLockedSetting(s.rules, key));
 }
 
+/**
+ * Whether the active account's webmail runs without third-party traffic
+ * (webmail hooks/use-third-party-mode.ts): remote content in mail never
+ * loads, whatever the setting or a trusted sender says.
+ */
+export function useNoThirdPartyTraffic(): boolean {
+  return usePolicyStore((s) => s.links.noThirdPartyTraffic === true);
+}
+
 /** Whether the active account's policy hides a setting (by the app's key). */
 export function useSettingHidden(key: string): boolean {
   return usePolicyStore((s) => s.rules.hidden.includes(key));