diff --git a/app/api/auth/session/route.ts b/app/api/auth/session/route.ts
index 3b0bbb5dd..6a0ce0a23 100644
--- a/app/api/auth/session/route.ts
+++ b/app/api/auth/session/route.ts
@@ -23,6 +23,7 @@ import { revokeWopiTokens } from '@/lib/wopi/revocation';
 import { clearPairReauthInStore } from '@/lib/auth/pair-reauth';
 import { loginClientAddress } from '@/lib/auth/client-address';
 import { LOGIN_BUDGET_RETRY_AFTER_SECONDS, reserveLoginAttempt } from '@/lib/auth/verify-budget';
+import { isPastMaxAge, maxSessionHoursFor, SESSION_TOO_OLD } from '@/lib/auth/session-age';
 
 function sessionCookieOptions() {
   return {
@@ -73,6 +74,42 @@ function getSlot(request: NextRequest): number {
   return slot;
 }
 
+/**
+ * The organisation's maximum session length for a remembered session
+ * (lib/auth/session-age.ts). A cookie from before sign-in times were
+ * recorded counts as signed in now and gets the time written into it.
+ * Returns the sign-in time, or null when the session is too old: its
+ * cookies are cleared then.
+ */
+async function checkSessionAge(
+  cookieStore: Awaited<ReturnType<typeof cookies>>,
+  slot: number,
+  credentials: SessionCredentials,
+): Promise<{ signedInAt: number } | { maxHours: number }> {
+  const cookieName = sessionCookieName(slot);
+  if (credentials.signedInAt === undefined) {
+    const signedInAt = Date.now();
+    const secret = credentials.token !== undefined ? { token: credentials.token } : credentials.password;
+    cookieStore.set(cookieName, encryptSession(credentials.serverUrl, credentials.username, secret, signedInAt), sessionCookieOptions());
+    return { signedInAt };
+  }
+  // An impersonation login is `target%master`; the policy is the target's.
+  const maxHours = await maxSessionHoursFor(credentials.username.split('%', 1)[0]);
+  if (isPastMaxAge(credentials.signedInAt, maxHours)) {
+    cookieStore.delete(cookieName);
+    clearStalwartAuthContextInStore(cookieStore, slot);
+    return { maxHours };
+  }
+  return { signedInAt: credentials.signedInAt };
+}
+
+function sessionTooOld(maxHours: number) {
+  return NextResponse.json(
+    { error: SESSION_TOO_OLD, maxHours },
+    { status: 401, headers: { 'Cache-Control': 'no-store, no-cache, must-revalidate' } },
+  );
+}
+
 export async function POST(request: NextRequest) {
   // CSRF gate (GHSA-qvr9-m8cq-7wvg): cookies written here are SameSite=Lax,
   // so a cross-site top-level POST would otherwise reach this handler.
@@ -162,7 +199,9 @@ export async function POST(request: NextRequest) {
       throw error;
     }
     refund();
-    const token = encryptSession(normalizedServerUrl, username, accessToken ? { token: accessToken } : password);
+    // A remembered sign-in starts the maximum session length.
+    const signedInAt = Date.now();
+    const token = encryptSession(normalizedServerUrl, username, accessToken ? { token: accessToken } : password, signedInAt);
     const cookieStore = await cookies();
     cookieStore.set(cookieName, token, sessionCookieOptions());
     setStalwartAuthContextInStore(cookieStore, slot, {
@@ -170,6 +209,7 @@ export async function POST(request: NextRequest) {
       username,
       authHeader,
       ...(accountName !== username ? { accountName } : {}),
+      signedInAt,
     });
 
     void recordLogin(username, normalizedServerUrl);
@@ -208,10 +248,14 @@ export async function GET(request: NextRequest) {
       return NextResponse.json({ error: 'Invalid session' }, { status: 401 });
     }
 
+    const age = await checkSessionAge(cookieStore, slot, credentials);
+    if ('maxHours' in age) return sessionTooOld(age.maxHours);
+
     setStalwartAuthContextInStore(cookieStore, slot, {
       serverUrl: credentials.serverUrl,
       username: credentials.username,
       authHeader: sessionAuthHeader(credentials),
+      signedInAt: age.signedInAt,
     });
 
     // Only return non-sensitive fields. Use PUT to retrieve full credentials.
@@ -258,13 +302,19 @@ export async function PUT(request: NextRequest) {
       return NextResponse.json({ error: 'Invalid session' }, { status: 401 });
     }
 
+    const age = await checkSessionAge(cookieStore, slot, credentials);
+    if ('maxHours' in age) return sessionTooOld(age.maxHours);
+
     setStalwartAuthContextInStore(cookieStore, slot, {
       serverUrl: credentials.serverUrl,
       username: credentials.username,
       authHeader: sessionAuthHeader(credentials),
+      signedInAt: age.signedInAt,
     });
 
-    return NextResponse.json(credentials, {
+    // The credentials as they were stored, without the bookkeeping.
+    const { signedInAt: _signedInAt, ...restored } = credentials;
+    return NextResponse.json(restored, {
       headers: { 'Cache-Control': 'no-store, no-cache, must-revalidate' },
     });
   } catch (error) {
diff --git a/app/api/auth/sso/complete/route.ts b/app/api/auth/sso/complete/route.ts
index de4a8f8de..5ae1dc11c 100644
--- a/app/api/auth/sso/complete/route.ts
+++ b/app/api/auth/sso/complete/route.ts
@@ -14,6 +14,7 @@ import { getCookieOptions } from '@/lib/oauth/cookie-config';
 import { storeIdToken } from '@/lib/oauth/end-session';
 import { rejectCrossOriginRequest } from '@/lib/security/same-origin';
 import { MAX_ACCOUNT_SLOTS } from '@/lib/account-utils';
+import { writeSignInStamp } from '@/lib/auth/session-age';
 
 const SSO_PENDING_COOKIE = 'sso_pending';
 const SSO_PENDING_MAX_AGE_MS = 5 * 60 * 1000; // 5 minutes
@@ -115,6 +116,8 @@ export async function POST(request: NextRequest) {
       if (tokens.refresh_token) {
         const cookieName = refreshTokenCookieName(slot);
         cookieStore.set(cookieName, tokens.refresh_token, getCookieOptions());
+        // A sign-in: the maximum session length counts from here (lib/auth/session-age.ts).
+        writeSignInStamp(cookieStore, slot, { signedInAt: Date.now() });
       }
       const serverCookieName = refreshTokenServerCookieName(slot);
       if (pendingServerId) {
diff --git a/app/api/auth/stalwart-context/route.ts b/app/api/auth/stalwart-context/route.ts
index 0d07ec38b..43c884d1e 100644
--- a/app/api/auth/stalwart-context/route.ts
+++ b/app/api/auth/stalwart-context/route.ts
@@ -1,7 +1,9 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { logger } from '@/lib/logger';
 import { JmapAuthVerificationError, resolveJmapIdentity } from '@/lib/auth/verify-jmap-auth';
-import { setStalwartAuthContext } from '@/lib/stalwart/auth-context';
+import { cookies } from 'next/headers';
+import { readStalwartAuthContextFromStore, setStalwartAuthContext } from '@/lib/stalwart/auth-context';
+import { readSignInStamp, writeSignInStamp } from '@/lib/auth/session-age';
 import { configManager } from '@/lib/admin/config-manager';
 import { isPublicHttpUrl } from '@/lib/security/url-guard';
 import { recordLogin } from '@/lib/telemetry/login-tracker';
@@ -101,11 +103,24 @@ export async function POST(request: NextRequest) {
     // Just signed in: an older refusal of this credential no longer applies.
     recordStoredCredentialStatus(authHeader, 200);
 
+    // The context is written again on every renewed token and restore, so
+    // the sign-in time is carried over: from this slot's context for the
+    // same account, else from the token sign-in's stamp, else it is now.
+    const cookieStore = await cookies();
+    const previous = readStalwartAuthContextFromStore(cookieStore, slot);
+    const stamp = readSignInStamp(cookieStore, slot);
+    const ownStamp = stamp && (!stamp.account || stamp.account === accountName) ? stamp : null;
+    const samePrevious = previous && previous.serverUrl === normalizedServerUrl && previous.username === username ? previous : null;
+    const signedInAt = samePrevious?.signedInAt ?? ownStamp?.signedInAt ?? Date.now();
+    // The token routes know no account; the first verified one is the stamp's.
+    if (ownStamp && !ownStamp.account) writeSignInStamp(cookieStore, slot, { ...ownStamp, account: accountName });
+
     await setStalwartAuthContext(slot, {
       serverUrl: normalizedServerUrl,
       username,
       authHeader,
       ...(accountName !== username ? { accountName } : {}),
+      signedInAt,
     });
 
     void recordLogin(username, normalizedServerUrl);
diff --git a/app/api/auth/token/route.ts b/app/api/auth/token/route.ts
index 05e7ec29c..942514fde 100644
--- a/app/api/auth/token/route.ts
+++ b/app/api/auth/token/route.ts
@@ -30,6 +30,15 @@ import {
 } from '@/lib/oauth/end-session';
 import { MAX_ACCOUNT_SLOTS } from '@/lib/account-utils';
 import { rejectCrossOriginRequest } from '@/lib/security/same-origin';
+import {
+  clearSignInStamp,
+  isPastMaxAge,
+  maxSessionHoursFor,
+  readSignInStamp,
+  SESSION_TOO_OLD,
+  writeSignInStamp,
+} from '@/lib/auth/session-age';
+import { clearStalwartAuthContextInStore, readStalwartAuthContextFromStore } from '@/lib/stalwart/auth-context';
 
 function parseSlot(raw: string | null): number | null {
   if (raw === null) return null;
@@ -152,6 +161,8 @@ export async function POST(request: NextRequest) {
     if (tokens.refresh_token) {
       const cookieName = refreshTokenCookieName(slot);
       cookieStore.set(cookieName, tokens.refresh_token, getCookieOptions());
+      // A sign-in: the maximum session length counts from here.
+      writeSignInStamp(cookieStore, slot, { signedInAt: Date.now() });
     }
     const cachedAccessToken = cacheAccessToken(cookieStore, slot, tokens.access_token, tokens.expires_in || 3600, [tokens.refresh_token, serverId]);
     storeIdToken(cookieStore, slot, tokens.id_token, request.nextUrl.basePath, [tokens.refresh_token, cachedAccessToken, serverId]);
@@ -189,6 +200,22 @@ export async function PUT(request: NextRequest) {
       return NextResponse.json({ error: 'No refresh token' }, { status: 401 });
     }
 
+    // The organisation's maximum session length (lib/auth/session-age.ts).
+    // A refresh token from before sign-in times were recorded counts as
+    // signed in now.
+    const stamp = readSignInStamp(cookieStore, slot);
+    if (!stamp) {
+      writeSignInStamp(cookieStore, slot, { signedInAt: Date.now() });
+    } else {
+      const context = readStalwartAuthContextFromStore(cookieStore, slot);
+      const maxHours = await maxSessionHoursFor(stamp.account ?? context?.accountName ?? context?.username ?? null);
+      if (isPastMaxAge(stamp.signedInAt, maxHours)) {
+        await signOutSlot(cookieStore, slot, request.nextUrl.basePath, false);
+        clearStalwartAuthContextInStore(cookieStore, slot);
+        return NextResponse.json({ error: SESSION_TOO_OLD, maxHours }, { status: 401 });
+      }
+    }
+
     // A session restore calls this to get its token back, not because the
     // current one expired. Serving the cached token avoids spending a refresh
     // the IdP may legitimately reject: Rauthy stamps refresh tokens with
@@ -307,6 +334,7 @@ async function signOutSlot(
   for (const name of [cookieName, refreshTokenServerCookieName(slot), accessTokenCookieName(slot)]) {
     if (cookieStore.get(name)) cookieStore.delete(name);
   }
+  clearSignInStamp(cookieStore, slot);
   if (idToken !== undefined) clearIdToken(cookieStore, slot, basePath);
   return endSessionUrl;
 }
diff --git a/app/api/auth/totp-token-exchange/route.ts b/app/api/auth/totp-token-exchange/route.ts
index 6172b7283..bdceadbf7 100644
--- a/app/api/auth/totp-token-exchange/route.ts
+++ b/app/api/auth/totp-token-exchange/route.ts
@@ -14,6 +14,7 @@ import { rejectCrossOriginRequest } from '@/lib/security/same-origin';
 import { stalwartPasswordLogin } from '@/lib/auth/stalwart-password-login';
 import { loginClientAddress } from '@/lib/auth/client-address';
 import { LOGIN_BUDGET_RETRY_AFTER_SECONDS, reserveLoginAttempt } from '@/lib/auth/verify-budget';
+import { writeSignInStamp } from '@/lib/auth/session-age';
 
 /**
  * Exchange a password + (optional) TOTP code for OAuth tokens.
@@ -213,6 +214,8 @@ async function storeAndRespond(
     // Without "Remember me" the sign-in ends with the browser session.
     const { maxAge: _maxAge, ...sessionOptions } = getCookieOptions();
     cookieStore.set(cookieName, tokens.refresh_token, rememberMe ? getCookieOptions() : sessionOptions);
+    // A sign-in: the maximum session length counts from here (lib/auth/session-age.ts).
+    writeSignInStamp(cookieStore, slot, { signedInAt: Date.now() });
   }
   const serverCookieName = refreshTokenServerCookieName(slot);
   if (serverId) {
diff --git a/lib/__tests__/auth-crypto.test.ts b/lib/__tests__/auth-crypto.test.ts
index 2fa3c854d..e30d869b5 100644
--- a/lib/__tests__/auth-crypto.test.ts
+++ b/lib/__tests__/auth-crypto.test.ts
@@ -31,6 +31,7 @@ describe('encryptSession / decryptSession', () => {
       serverUrl: 'https://mail.example.com',
       username: 'alice',
       password: 's3cret',
+      signedInAt: expect.any(Number),
     });
   });
 
diff --git a/lib/__tests__/auth-login-routes-budget.test.ts b/lib/__tests__/auth-login-routes-budget.test.ts
index 793cc52f5..1a228c07a 100644
--- a/lib/__tests__/auth-login-routes-budget.test.ts
+++ b/lib/__tests__/auth-login-routes-budget.test.ts
@@ -48,6 +48,7 @@ vi.mock('@/lib/stalwart/auth-context', () => ({
   setStalwartAuthContext: vi.fn(),
   setStalwartAuthContextInStore: vi.fn(),
   clearStalwartAuthContextInStore: vi.fn(),
+  readStalwartAuthContextFromStore: () => null,
 }));
 
 vi.mock('@/lib/telemetry/login-tracker', () => ({ recordLogin: vi.fn() }));
diff --git a/lib/__tests__/auth-routes-credential-check.test.ts b/lib/__tests__/auth-routes-credential-check.test.ts
index 3c49cbe32..f286fc472 100644
--- a/lib/__tests__/auth-routes-credential-check.test.ts
+++ b/lib/__tests__/auth-routes-credential-check.test.ts
@@ -56,6 +56,7 @@ vi.mock('@/lib/stalwart/auth-context', () => ({
   setStalwartAuthContext: (...args: unknown[]) => setStalwartAuthContext(...args),
   setStalwartAuthContextInStore: (...args: unknown[]) => setStalwartAuthContextInStore(...args),
   clearStalwartAuthContextInStore: vi.fn(),
+  readStalwartAuthContextFromStore: () => null,
 }));
 
 const recordLogin = vi.fn();
diff --git a/lib/__tests__/session-max-age-routes.test.ts b/lib/__tests__/session-max-age-routes.test.ts
new file mode 100644
index 000000000..5b3dd23b8
--- /dev/null
+++ b/lib/__tests__/session-max-age-routes.test.ts
@@ -0,0 +1,240 @@
+// @vitest-environment node
+import { createCipheriv, createHash, randomBytes } from 'node:crypto';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { NextRequest } from 'next/server';
+import type { SettingsPolicy } from '@/lib/admin/types';
+
+/**
+ * The organisation's maximum session length on the server: the routes that
+ * restore a remembered sign-in or renew a provider token refuse a session
+ * older than `maxHours` of the account's domain scopes, and a cookie from
+ * before sign-in times were recorded counts as signed in when first seen.
+ */
+
+const SECRET = 's'.repeat(64);
+vi.mock('@/lib/auth/session-secret', () => ({ getSessionSecret: () => SECRET, hasSessionSecret: () => true }));
+vi.mock('@/lib/logger', () => ({ logger: { warn: () => {}, error: () => {}, info: () => {}, debug: () => {} } }));
+vi.mock('@/lib/telemetry/login-tracker', () => ({ recordLogin: () => {} }));
+vi.mock('@/lib/oauth/cookie-config', () => ({
+  getCookieOptions: () => ({ httpOnly: true, secure: true, sameSite: 'lax', path: '/', maxAge: 2592000 }),
+}));
+
+const jar = new Map<string, string>();
+vi.mock('next/headers', () => ({
+  cookies: async () => ({
+    get: (name: string) => (jar.has(name) ? { name, value: jar.get(name)! } : undefined),
+    set: (name: string, value: string) => { jar.set(name, value); },
+    delete: (arg: string | { name: string }) => { jar.delete(typeof arg === 'string' ? arg : arg.name); },
+    getAll: () => [...jar].map(([name, value]) => ({ name, value })),
+  }),
+}));
+
+let policy: Partial<SettingsPolicy>;
+vi.mock('@/lib/admin/config-manager', () => ({
+  configManager: {
+    ensureLoaded: async () => {},
+    get: (key: string, fallback: unknown) => (key === 'jmapServerUrl' ? 'https://mail.example.org' : fallback),
+    getPolicy: () => policy,
+  },
+}));
+
+vi.mock('@/lib/auth/verify-jmap-auth', () => ({
+  JmapAuthVerificationError: class extends Error { status = 401; },
+  resolveJmapIdentity: async (serverUrl: string, _auth: string, username: string) => ({ serverUrl, accountName: username }),
+}));
+
+const tokenFetch = vi.fn();
+vi.mock('@/lib/oauth/token-exchange', () => ({
+  exchangeCodeForTokens: async () => ({ access_token: 'first-access', refresh_token: 'refresh-1', expires_in: 300 }),
+  buildOAuthParams: (params: Record<string, string>) => new URLSearchParams(params),
+  getMetadata: async () => ({ revocation_endpoint: 'https://id.example.org/revoke' }),
+  getRequiredConfig: () => ({ clientId: 'webmail' }),
+  getTokenEndpoint: async () => 'https://id.example.org/token',
+  defaultClientOptions: async () => ({ fallbackClientId: 'bulwark-webmail' }),
+  DEFAULT_CLIENT_ID: 'bulwark-webmail',
+}));
+
+const { encryptSession, decryptSession, decryptPayload } = await import('@/lib/auth/crypto');
+const { readSignInStamp, writeSignInStamp } = await import('@/lib/auth/session-age');
+const sessionRoute = await import('@/app/api/auth/session/route');
+const tokenRoute = await import('@/app/api/auth/token/route');
+const contextRoute = await import('@/app/api/auth/stalwart-context/route');
+
+const HOUR = 60 * 60 * 1000;
+const NOW = new Date('2026-10-28T12:00:00Z').getTime();
+const SERVER = 'https://mail.example.org';
+const SAME_ORIGIN = { 'sec-fetch-site': 'same-origin', 'sec-fetch-mode': 'cors', 'sec-fetch-dest': 'empty' };
+
+/** Eight hours for example.org, nothing for everyone else. */
+function policyWithDomainMax(hours: number): Partial<SettingsPolicy> {
+  return {
+    features: {} as never, restrictions: {}, defaults: {},
+    scopes: [{ id: 'example', label: 'example.org', match: { domains: ['example.org'] }, policy: { organisation: { session: { maxHours: hours } } } }],
+  };
+}
+
+/** A remembered-session cookie as Bulwark wrote it before sign-in times were recorded. */
+function legacySessionCookie(username: string, password: string): string {
+  const key = createHash('sha256').update(SECRET).digest();
+  const iv = randomBytes(12);
+  const cipher = createCipheriv('aes-256-gcm', key, iv);
+  const encrypted = Buffer.concat([cipher.update(JSON.stringify({ v: 1, serverUrl: SERVER, username, password }), 'utf8'), cipher.final()]);
+  return Buffer.concat([iv, cipher.getAuthTag(), encrypted]).toString('base64');
+}
+
+const restore = () => sessionRoute.PUT(new NextRequest('https://webmail.example/api/auth/session', { method: 'PUT', headers: SAME_ORIGIN }));
+const peek = () => sessionRoute.GET(new NextRequest('https://webmail.example/api/auth/session'));
+const refresh = () => tokenRoute.PUT(new NextRequest('https://webmail.example/api/auth/token?slot=0', { method: 'PUT', headers: SAME_ORIGIN }));
+const context = () => decryptPayload(jar.get('jmap_stalwart_ctx')!, 'session-context');
+
+beforeEach(() => {
+  jar.clear();
+  vi.useFakeTimers({ toFake: ['Date'] });
+  vi.setSystemTime(NOW);
+  policy = policyWithDomainMax(8);
+  tokenFetch.mockReset();
+  tokenFetch.mockImplementation(async (url: string) => {
+    if (url.endsWith('/token')) return Response.json({ access_token: 'new-access', expires_in: 300 });
+    return new Response(null, { status: 200 });
+  });
+  vi.stubGlobal('fetch', tokenFetch);
+});
+
+afterEach(() => {
+  vi.useRealTimers();
+  vi.unstubAllGlobals();
+});
+
+describe('remembered sessions (/api/auth/session)', () => {
+  it('records the sign-in time when the session is remembered', async () => {
+    const res = await sessionRoute.POST(new NextRequest('https://webmail.example/api/auth/session?slot=0', {
+      method: 'POST',
+      headers: { ...SAME_ORIGIN, 'content-type': 'application/json' },
+      body: JSON.stringify({ serverUrl: SERVER, username: 'org-alice@example.org', password: 'pw' }),
+    }));
+    expect(res.status).toBe(200);
+    expect(decryptSession(jar.get('jmap_session')!)?.signedInAt).toBe(NOW);
+    expect(context()?.signedInAt).toBe(NOW);
+  });
+
+  it('restores a session younger than the maximum, without the sign-in time in the answer', async () => {
+    jar.set('jmap_session', encryptSession(SERVER, 'org-alice@example.org', 'pw', NOW - 7 * HOUR));
+    const res = await restore();
+    expect(res.status).toBe(200);
+    expect(await res.json()).toEqual({ serverUrl: SERVER, username: 'org-alice@example.org', password: 'pw' });
+    expect(context()?.signedInAt).toBe(NOW - 7 * HOUR);
+  });
+
+  it('refuses a session older than its domain allows and clears its cookies', async () => {
+    jar.set('jmap_session', encryptSession(SERVER, 'org-alice@example.org', 'pw', NOW - 8 * HOUR));
+    jar.set('jmap_stalwart_ctx', 'stale');
+
+    const res = await restore();
+    expect(res.status).toBe(401);
+    expect(await res.json()).toEqual({ error: 'session_expired', maxHours: 8 });
+    expect(jar.has('jmap_session')).toBe(false);
+    expect(jar.has('jmap_stalwart_ctx')).toBe(false);
+
+    jar.set('jmap_session', encryptSession(SERVER, 'org-alice@example.org', 'pw', NOW - 9 * HOUR));
+    expect((await peek()).status).toBe(401);
+  });
+
+  it('leaves an account of another domain alone', async () => {
+    jar.set('jmap_session', encryptSession(SERVER, 'org-dave@org-two.test', 'pw', NOW - 100 * HOUR));
+    expect((await restore()).status).toBe(200);
+  });
+
+  it('applies the maximum of Everyone to every domain', async () => {
+    policy = { ...policyWithDomainMax(8), organisation: { session: { idleMinutes: 0, maxHours: 24, warnSeconds: 60 } } as never };
+    jar.set('jmap_session', encryptSession(SERVER, 'org-dave@org-two.test', 'pw', NOW - 25 * HOUR));
+    expect((await restore()).status).toBe(401);
+  });
+
+  it('counts a cookie without a sign-in time as signed in when first seen', async () => {
+    jar.set('jmap_session', legacySessionCookie('org-alice@example.org', 'pw'));
+
+    const first = await restore();
+    expect(first.status).toBe(200);
+    expect(await first.json()).toEqual({ serverUrl: SERVER, username: 'org-alice@example.org', password: 'pw' });
+    expect(decryptSession(jar.get('jmap_session')!)?.signedInAt).toBe(NOW);
+
+    vi.setSystemTime(NOW + 7 * HOUR);
+    expect((await restore()).status).toBe(200);
+    vi.setSystemTime(NOW + 8 * HOUR);
+    expect((await restore()).status).toBe(401);
+  });
+});
+
+describe('provider tokens (/api/auth/token)', () => {
+  it('records the sign-in time when a provider sign-in stores its refresh token', async () => {
+    // A leftover stamp from an earlier sign-in in this slot.
+    const store = (await (await import('next/headers')).cookies()) as never;
+    writeSignInStamp(store, 0, { signedInAt: NOW - 20 * HOUR, account: 'org-bob@example.org' });
+
+    const res = await tokenRoute.POST(new NextRequest('https://webmail.example/api/auth/token?slot=0', {
+      method: 'POST',
+      headers: { ...SAME_ORIGIN, 'content-type': 'application/json' },
+      body: JSON.stringify({ code: 'c', code_verifier: 'v', redirect_uri: 'https://webmail.example/auth/callback', slot: 0 }),
+    }));
+    expect(res.status).toBe(200);
+    expect(readSignInStamp(store, 0)).toEqual({ signedInAt: NOW });
+  });
+
+  it('refuses to renew a token sign-in older than the maximum and revokes it', async () => {
+    jar.set('jmap_rt', 'refresh-1');
+    const store = (await (await import('next/headers')).cookies()) as never;
+    writeSignInStamp(store, 0, { signedInAt: NOW - 8 * HOUR, account: 'org-alice@example.org' });
+
+    const res = await refresh();
+    expect(res.status).toBe(401);
+    expect(await res.json()).toEqual({ error: 'session_expired', maxHours: 8 });
+    expect(jar.has('jmap_rt')).toBe(false);
+    expect(jar.has('jmap_signed_in')).toBe(false);
+    expect(tokenFetch).toHaveBeenCalledWith('https://id.example.org/revoke', expect.anything());
+    expect(tokenFetch).not.toHaveBeenCalledWith('https://id.example.org/token', expect.anything());
+  });
+
+  it('renews a token sign-in younger than the maximum', async () => {
+    jar.set('jmap_rt', 'refresh-1');
+    const store = (await (await import('next/headers')).cookies()) as never;
+    writeSignInStamp(store, 0, { signedInAt: NOW - 7 * HOUR, account: 'org-alice@example.org' });
+
+    const res = await refresh();
+    expect(res.status).toBe(200);
+    expect((await res.json()).access_token).toBe('new-access');
+  });
+
+  it('counts a refresh token without a sign-in time as signed in when first seen', async () => {
+    jar.set('jmap_rt', 'refresh-1');
+
+    expect((await refresh()).status).toBe(200);
+    const store = (await (await import('next/headers')).cookies()) as never;
+    expect(readSignInStamp(store, 0)).toEqual({ signedInAt: NOW });
+  });
+});
+
+describe('the auth context (/api/auth/stalwart-context)', () => {
+  const post = () => contextRoute.POST(new NextRequest('https://webmail.example/api/auth/stalwart-context', {
+    method: 'POST',
+    headers: { ...SAME_ORIGIN, 'content-type': 'application/json' },
+    body: JSON.stringify({ serverUrl: SERVER, username: 'org-alice@example.org', authHeader: 'Bearer new', slot: 0 }),
+  }));
+
+  it('keeps the sign-in time when a renewed token is written again', async () => {
+    jar.set('jmap_session', encryptSession(SERVER, 'org-alice@example.org', 'pw', NOW - 3 * HOUR));
+    await restore();
+    vi.setSystemTime(NOW + HOUR);
+
+    expect((await post()).status).toBe(200);
+    expect(context()?.signedInAt).toBe(NOW - 3 * HOUR);
+  });
+
+  it("takes a token sign-in's time from its stamp and names the account on it", async () => {
+    const store = (await (await import('next/headers')).cookies()) as never;
+    writeSignInStamp(store, 0, { signedInAt: NOW - 2 * HOUR });
+
+    expect((await post()).status).toBe(200);
+    expect(context()?.signedInAt).toBe(NOW - 2 * HOUR);
+    expect(readSignInStamp(store, 0)).toEqual({ signedInAt: NOW - 2 * HOUR, account: 'org-alice@example.org' });
+  });
+});
diff --git a/lib/auth/crypto.ts b/lib/auth/crypto.ts
index 1dfc768ab..89b2545a9 100644
--- a/lib/auth/crypto.ts
+++ b/lib/auth/crypto.ts
@@ -23,10 +23,13 @@ function getKey(): Buffer {
 /**
  * A remembered sign-in: the account's password (Basic auth), or the access
  * token the user signed in with (Bearer auth, e.g. a Fastmail API token).
+ * `signedInAt`: when the user signed in (ms), for the organisation's maximum
+ * session length (lib/auth/session-age.ts); absent in cookies written
+ * before it was recorded.
  */
 export type SessionCredentials =
-  | { serverUrl: string; username: string; password: string; token?: undefined }
-  | { serverUrl: string; username: string; token: string; password?: undefined };
+  | { serverUrl: string; username: string; password: string; token?: undefined; signedInAt?: number }
+  | { serverUrl: string; username: string; token: string; password?: undefined; signedInAt?: number };
 
 /** The Authorization header remembered credentials sign in with. */
 export function sessionAuthHeader(credentials: SessionCredentials): string {
@@ -34,13 +37,18 @@ export function sessionAuthHeader(credentials: SessionCredentials): string {
   return `Basic ${Buffer.from(`${credentials.username}:${credentials.password}`).toString('base64')}`;
 }
 
-export function encryptSession(serverUrl: string, username: string, secret: string | { token: string }): string {
+export function encryptSession(
+  serverUrl: string,
+  username: string,
+  secret: string | { token: string },
+  signedInAt: number = Date.now(),
+): string {
   const key = getKey();
   const iv = randomBytes(IV_LENGTH);
   const cipher = createCipheriv(ALGORITHM, key, iv);
 
   const credential = typeof secret === 'string' ? { password: secret } : { token: secret.token };
-  const payload = JSON.stringify({ v: 1, serverUrl, username, ...credential });
+  const payload = JSON.stringify({ v: 1, serverUrl, username, ...credential, signedInAt });
   const encrypted = Buffer.concat([cipher.update(payload, 'utf8'), cipher.final()]);
   const tag = cipher.getAuthTag();
 
@@ -64,11 +72,14 @@ export function decryptSession(token: string): SessionCredentials | null {
     const parsed = JSON.parse(decrypted.toString('utf8'));
 
     if (parsed.v !== 1 || !parsed.serverUrl || !parsed.username) return null;
+    const signedInAt = typeof parsed.signedInAt === 'number' && Number.isFinite(parsed.signedInAt) && parsed.signedInAt > 0
+      ? { signedInAt: parsed.signedInAt as number }
+      : {};
     if (typeof parsed.password === 'string' && parsed.password) {
-      return { serverUrl: parsed.serverUrl, username: parsed.username, password: parsed.password };
+      return { serverUrl: parsed.serverUrl, username: parsed.username, password: parsed.password, ...signedInAt };
     }
     if (typeof parsed.token === 'string' && parsed.token) {
-      return { serverUrl: parsed.serverUrl, username: parsed.username, token: parsed.token };
+      return { serverUrl: parsed.serverUrl, username: parsed.username, token: parsed.token, ...signedInAt };
     }
     return null;
   } catch (error) {
@@ -104,7 +115,9 @@ export type PayloadPurpose =
   /** The app password an impersonation handoff minted, kept for revocation. */
   | 'impersonation-grant'
   /** A user's plugin credentials (lib/plugin-secrets/vault.ts). */
-  | 'plugin-secrets';
+  | 'plugin-secrets'
+  /** When a token sign-in happened, for the maximum session length (lib/auth/session-age.ts). */
+  | 'session-age';
 
 /**
  * Whether a blob minted before purposes were bound may be re-admitted for
@@ -125,6 +138,7 @@ const LEGACY_PAYLOAD_MARKERS: Record<PayloadPurpose, (payload: Record<string, un
   'wopi-token': (p) => p.t === 'wopi',
   'impersonation-grant': () => false,
   'plugin-secrets': () => false,
+  'session-age': () => false,
 };
 
 export function encryptPayload(payload: Record<string, unknown>, purpose: PayloadPurpose): string {
diff --git a/lib/auth/session-age.ts b/lib/auth/session-age.ts
new file mode 100644
index 000000000..e5689e889
--- /dev/null
+++ b/lib/auth/session-age.ts
@@ -0,0 +1,102 @@
+/**
+ * The organisation's maximum session length (policy.json
+ * `organisation.session.maxHours`) on the server side.
+ *
+ * JMAP goes from the browser straight to Stalwart, so the client's timer
+ * signs users out on time (components/session/session-timeout-guard.tsx).
+ * The server's part is to stop a session older than the maximum from coming
+ * back: the routes that restore a remembered sign-in (/api/auth/session) or
+ * renew a provider token (/api/auth/token) refuse it and clear its cookies.
+ *
+ * The sign-in time travels in the remembered-session cookie (`signedInAt`
+ * in lib/auth/crypto.ts), in the auth-context cookie, and for token
+ * sign-ins in a small cookie of its own written when the refresh token is
+ * (the auth-context cookie ends with the browser, the refresh token lasts
+ * 30 days). A cookie written before this existed has no sign-in time: it
+ * counts as signed in when the server first sees it after the upgrade,
+ * which then writes the time into it.
+ *
+ * The server knows an account's address, not its groups, so only Everyone
+ * and the domain scopes count here; a group scope's value is enforced by the
+ * client alone.
+ */
+
+import type { cookies } from 'next/headers';
+import { configManager } from '@/lib/admin/config-manager';
+import { resolvePolicy } from '@/lib/admin/policy-scopes';
+import { decryptPayload, encryptPayload } from '@/lib/auth/crypto';
+import { getCookieOptions } from '@/lib/oauth/cookie-config';
+import { logger } from '@/lib/logger';
+
+type CookieStore = Awaited<ReturnType<typeof cookies>>;
+
+const HOUR_MS = 60 * 60 * 1000;
+
+/** What a route answers when it refuses a session that is too old. */
+export const SESSION_TOO_OLD = 'session_expired';
+
+/** The maximum session length (hours, 0 = none) for an account's address. */
+export async function maxSessionHoursFor(account: string | null | undefined): Promise<number> {
+  try {
+    await configManager.ensureLoaded?.();
+    const email = account?.trim().toLowerCase() ?? '';
+    const policy = resolvePolicy(configManager.getPolicy(), email ? { email, groups: [] } : null).policy;
+    const hours = policy.organisation?.session?.maxHours;
+    return typeof hours === 'number' && hours > 0 ? hours : 0;
+  } catch (error) {
+    logger.warn('Could not read the maximum session length', { error: error instanceof Error ? error.message : 'Unknown error' });
+    return 0;
+  }
+}
+
+/** Whether a session signed in at `signedInAt` has outlived `maxHours`. */
+export function isPastMaxAge(signedInAt: number | undefined, maxHours: number, now = Date.now()): boolean {
+  if (!maxHours || typeof signedInAt !== 'number') return false;
+  return now - signedInAt >= maxHours * HOUR_MS;
+}
+
+export function validSignedInAt(value: unknown): number | undefined {
+  return typeof value === 'number' && Number.isFinite(value) && value > 0 ? value : undefined;
+}
+
+/**
+ * The sign-in time of a token sign-in, kept as long as its refresh token.
+ * `account` is filled in once the auth context names the account.
+ */
+export interface SignInStamp {
+  signedInAt: number;
+  account?: string;
+}
+
+export function signInStampCookieName(slot: number): string {
+  return slot === 0 ? 'jmap_signed_in' : `jmap_signed_in_${slot}`;
+}
+
+export function readSignInStamp(cookieStore: CookieStore, slot: number): SignInStamp | null {
+  const value = cookieStore.get(signInStampCookieName(slot))?.value;
+  if (!value) return null;
+  try {
+    const payload = decryptPayload(value, 'session-age');
+    const signedInAt = validSignedInAt(payload?.signedInAt);
+    if (!signedInAt) return null;
+    return typeof payload?.account === 'string' ? { signedInAt, account: payload.account } : { signedInAt };
+  } catch {
+    return null;
+  }
+}
+
+/** Without a session secret nothing can be sealed; the client's timer still applies. */
+export function writeSignInStamp(cookieStore: CookieStore, slot: number, stamp: SignInStamp): void {
+  try {
+    const payload: Record<string, unknown> = { signedInAt: stamp.signedInAt };
+    if (stamp.account) payload.account = stamp.account;
+    cookieStore.set(signInStampCookieName(slot), encryptPayload(payload, 'session-age'), getCookieOptions());
+  } catch (error) {
+    logger.warn('Could not record the sign-in time', { error: error instanceof Error ? error.message : 'Unknown error' });
+  }
+}
+
+export function clearSignInStamp(cookieStore: CookieStore, slot: number): void {
+  const name = signInStampCookieName(slot);
+  if (cookieStore.get(name)) cookieStore.delete(name);
+}
diff --git a/lib/stalwart/auth-context.ts b/lib/stalwart/auth-context.ts
index 7d22b3f63..3a34f1901 100644
--- a/lib/stalwart/auth-context.ts
+++ b/lib/stalwart/auth-context.ts
@@ -14,6 +14,12 @@ export interface StalwartAuthContext {
    * data such as synced settings is keyed on this.
    */
   accountName?: string;
+  /**
+   * When the user signed in (ms), for the organisation's maximum session
+   * length (lib/auth/session-age.ts). Carried over when the context is
+   * written again for the same account (a renewed token, a restore).
+   */
+  signedInAt?: number;
 }
 
 type CookieStore = Awaited<ReturnType<typeof cookies>>;
@@ -39,7 +45,8 @@ function isValidContext(payload: unknown): payload is StalwartAuthContext {
   return typeof candidate.serverUrl === 'string'
     && typeof candidate.username === 'string'
     && typeof candidate.authHeader === 'string'
-    && (candidate.accountName === undefined || typeof candidate.accountName === 'string');
+    && (candidate.accountName === undefined || typeof candidate.accountName === 'string')
+    && (candidate.signedInAt === undefined || typeof candidate.signedInAt === 'number');
 }
 
 function getSessionCookieOptions() {
@@ -59,8 +66,14 @@ export function readStalwartAuthContextFromStore(
 
   // Hand back exactly the declared shape: the envelope's own bookkeeping
   // stays in the envelope rather than riding along into the credentials.
-  const { serverUrl, username, authHeader, accountName } = payload;
-  return accountName ? { serverUrl, username, authHeader, accountName } : { serverUrl, username, authHeader };
+  const { serverUrl, username, authHeader, accountName, signedInAt } = payload;
+  return {
+    serverUrl,
+    username,
+    authHeader,
+    ...(accountName ? { accountName } : {}),
+    ...(typeof signedInAt === 'number' ? { signedInAt } : {}),
+  };
 }
 
 export function setStalwartAuthContextInStore(
diff --git a/stores/__tests__/auth-store-restore-session-too-old.test.ts b/stores/__tests__/auth-store-restore-session-too-old.test.ts
new file mode 100644
index 000000000..333775cf5
--- /dev/null
+++ b/stores/__tests__/auth-store-restore-session-too-old.test.ts
@@ -0,0 +1,64 @@
+import { beforeEach, expect, it, vi } from 'vitest';
+import { createConfig } from '@/lib/__tests__/fixtures/config';
+
+// The server refuses to restore a session older than the organisation's
+// maximum session length (lib/auth/session-age.ts). The login page then says
+// why the user has to sign in again.
+vi.mock('@/lib/jmap/client', () => {
+  class JMAPClient {
+    constructor(public serverUrl: string, public username: string, public password: string) {}
+    static withBearer(serverUrl: string, _token: string, username: string) { return new JMAPClient(serverUrl, username, ''); }
+    async connect() {}
+    getAuthHeader() { return 'Basic x'; }
+    onConnectionChange() {}
+    onRateLimit() {}
+    getRateLimitRemainingMs() { return 0; }
+    disconnect() {}
+  }
+  class RateLimitError extends Error {}
+  class RequestTimeoutError extends Error {}
+  return { JMAPClient, RateLimitError, RequestTimeoutError };
+});
+
+import * as nav from '@/lib/browser-navigation';
+import { useAuthStore } from '@/stores/auth-store';
+import { useAccountStore } from '@/stores/account-store';
+import { consumeSessionEnd } from '@/lib/session-policy/sign-out-reason';
+
+const account = (authMode: 'basic' | 'oauth') => ({
+  id: 'alice@example.org@mail.example.org', label: 'Alice', serverUrl: 'https://mail.example.org', username: 'alice@example.org', authMode,
+  rememberMe: true, providerSession: authMode === 'oauth', cookieSlot: 0, displayName: 'Alice', email: 'alice@example.org', isConnected: false,
+  hasError: false, lastLoginAt: 1, avatarColor: '#000000', isDefault: true,
+});
+
+const tooOld = () => new Response(JSON.stringify({ error: 'session_expired', maxHours: 8 }), { status: 401 });
+
+beforeEach(() => {
+  sessionStorage.clear();
+  vi.stubGlobal('fetch', vi.fn(async (input: unknown, init?: { method?: string }) => {
+    const url = String(input);
+    if (url.includes('/api/config')) return new Response(JSON.stringify(createConfig()), { status: 200 });
+    if (url.includes('/api/auth/session') && init?.method === 'PUT') return tooOld();
+    if (url.includes('/api/auth/token') && init?.method === 'PUT') return tooOld();
+    return new Response('{}', { status: 200 });
+  }));
+  vi.spyOn(nav, 'replaceWindowLocation').mockImplementation(() => {});
+});
+
+it('says on the login page that a remembered session reached its maximum length', async () => {
+  useAccountStore.setState({ accounts: [account('basic')] as never, activeAccountId: account('basic').id, defaultAccountId: account('basic').id });
+
+  await useAuthStore.getState().checkAuth();
+  await vi.waitFor(() => expect(useAccountStore.getState().accounts).toHaveLength(0));
+
+  expect(consumeSessionEnd()).toEqual({ kind: 'max', hours: 8 });
+});
+
+it('says so for a provider sign-in whose renewal the server refused', async () => {
+  useAccountStore.setState({ accounts: [account('oauth')] as never, activeAccountId: account('oauth').id, defaultAccountId: account('oauth').id });
+
+  await useAuthStore.getState().checkAuth();
+  await vi.waitFor(() => expect(useAccountStore.getState().accounts).toHaveLength(0));
+
+  expect(consumeSessionEnd()).toEqual({ kind: 'max', hours: 8 });
+});
diff --git a/stores/auth-store.ts b/stores/auth-store.ts
index 0a414af71..b8b084d5d 100644
--- a/stores/auth-store.ts
+++ b/stores/auth-store.ts
@@ -24,6 +24,7 @@ import { useSearchHistoryStore } from './search-history-store';
 import { useCalendarNotificationStore } from './calendar-notification-store';
 import { usePolicyStore } from './policy-store';
 import { policySubjectForClient } from '@/lib/admin/policy-subject';
+import { rememberSessionEnd } from '@/lib/session-policy/sign-out-reason';
 import type { Identity } from '@/lib/jmap/types';
 import { authHooks } from '@/lib/plugin-hooks';
 import { IS_LITE, IS_LITE_STALWART } from '@/lib/lite';
@@ -536,12 +537,31 @@ async function persistSession(slot: number, session: RememberedSession): Promise
   }
 }
 
+/**
+ * A restore or renewal the server refused because the session reached the
+ * organisation's maximum length (lib/auth/session-age.ts): the login page
+ * says so.
+ */
+async function noteSessionTooOld(res: Response): Promise<Response> {
+  if (res.status !== 401 || typeof res.clone !== 'function') return res;
+  try {
+    const data = await res.clone().json() as { error?: unknown; maxHours?: unknown };
+    if (data?.error === 'session_expired' && typeof data.maxHours === 'number') {
+      rememberSessionEnd({ kind: 'max', hours: data.maxHours });
+    }
+  } catch {
+    /* not JSON */
+  }
+  return res;
+}
+
 /** PUT /api/auth/token?slot=N - renew (or, unless `force`, reuse) the slot's access token. */
 async function fetchSlotAccessToken(slot: number, opts: { force?: boolean } = {}): Promise<Response> {
   // Renewing would write the tokens of a slot being signed out back.
   if (closingSlots.has(slot)) return jsonResponse({ error: 'signed_out' }, 401);
   if (!IS_LITE) {
-    return trackSessionWrite(slot, (signal) => apiFetch(`/api/auth/token?slot=${slot}${opts.force ? '&force=true' : ''}`, { method: 'PUT', signal }));
+    return trackSessionWrite(slot, (signal) => apiFetch(`/api/auth/token?slot=${slot}${opts.force ? '&force=true' : ''}`, { method: 'PUT', signal }))
+      .then(noteSessionTooOld);
   }
   // Like the route's cookie cache: a restore resumes with the token it had,
   // since a renewal before the refresh token's `nbf` is refused (#552).
@@ -563,7 +583,8 @@ async function fetchSlotAccessToken(slot: number, opts: { force?: boolean } = {}
 /** PUT /api/auth/session[?slot=N] - the slot's remembered password or access token. */
 async function fetchSlotSession(slot?: number): Promise<Response> {
   if (!IS_LITE) {
-    return apiFetch(slot === undefined ? '/api/auth/session' : `/api/auth/session?slot=${slot}`, { method: 'PUT' });
+    return apiFetch(slot === undefined ? '/api/auth/session' : `/api/auth/session?slot=${slot}`, { method: 'PUT' })
+      .then(noteSessionTooOld);
   }
   const stored = readLiteBasicSession(slot ?? 0);
   return stored ? jsonResponse(stored) : jsonResponse({ error: 'no_session' }, 401);