diff --git a/src/lib/__tests__/oauth.test.ts b/src/lib/__tests__/oauth.test.ts
index d31dd751..1bd42e92 100644
--- a/src/lib/__tests__/oauth.test.ts
+++ b/src/lib/__tests__/oauth.test.ts
@@ -8,11 +8,12 @@ vi.mock('../client-cert', () => ({
   secureFetch: mockSecureFetch,
 }));
 
-import { refreshOAuthAccessToken, type OAuthTokens } from '../oauth';
+import { refreshOAuthAccessToken, resetRefreshRotations, type OAuthTokens } from '../oauth';
 
 describe('oauth token refresh deduplication', () => {
   beforeEach(() => {
     vi.clearAllMocks();
+    resetRefreshRotations();
   });
 
   it('should only send a single HTTP request when refreshOAuthAccessToken is called concurrently with the same refresh token', async () => {
@@ -53,11 +54,58 @@ describe('oauth token refresh deduplication', () => {
     expect(res2.accessToken).toBe('new-access-token');
     expect(res2.refreshToken).toBe('rotated-refresh-token');
 
-    // Fire another refresh after the first completes (now the map should be empty for this token)
+    // A holder that still has the rotated-away token later gets its
+    // successor: presenting it again would revoke the grant on Stalwart 1.0.
     mockSecureFetch.mockClear();
     const res3 = await refreshOAuthAccessToken(tokens);
-    expect(mockSecureFetch).toHaveBeenCalledTimes(1);
+    expect(mockSecureFetch).not.toHaveBeenCalled();
     expect(res3.accessToken).toBe('new-access-token');
+    expect(res3.refreshToken).toBe('rotated-refresh-token');
+  });
+
+  it('refreshes again with the successor once its access token runs out, never with the old token', async () => {
+    const tokens: OAuthTokens = {
+      accessToken: 'a1',
+      refreshToken: 'r1',
+      tokenEndpoint: 'https://auth.example.com/token',
+      clientId: 'swc1.x',
+    };
+    let generation = 1;
+    mockSecureFetch.mockImplementation(async () => {
+      generation += 1;
+      return {
+        ok: true,
+        status: 200,
+        json: async () => ({ access_token: `a${generation}`, refresh_token: `r${generation}`, expires_in: 30 }),
+      } as Response;
+    });
+
+    await refreshOAuthAccessToken(tokens);
+    // 30 s left on the successor: not enough to hand out, so it is renewed.
+    const res = await refreshOAuthAccessToken(tokens);
+
+    const presented = mockSecureFetch.mock.calls.map(([, init]) => new URLSearchParams(String(init.body)).get('refresh_token'));
+    expect(presented).toEqual(['r1', 'r2']);
+    expect(res.refreshToken).toBe('r3');
+  });
+
+  it('keeps refreshing with the same token on a server that does not rotate', async () => {
+    const tokens: OAuthTokens = {
+      accessToken: 'a1',
+      refreshToken: 'stable',
+      tokenEndpoint: 'https://auth.example.com/token',
+      clientId: 'bulwark-webmail',
+    };
+    mockSecureFetch.mockImplementation(async () => ({
+      ok: true,
+      status: 200,
+      json: async () => ({ access_token: 'a2', expires_in: 3600 }),
+    }) as Response);
+
+    await refreshOAuthAccessToken(tokens);
+    await refreshOAuthAccessToken(tokens);
+
+    expect(mockSecureFetch).toHaveBeenCalledTimes(2);
   });
 
   it('should propagate errors to all waiters if the HTTP request fails', async () => {
diff --git a/src/lib/oauth.ts b/src/lib/oauth.ts
index a31db07c..d99abfd7 100644
--- a/src/lib/oauth.ts
+++ b/src/lib/oauth.ts
@@ -535,6 +535,45 @@ export async function redeemPairingCode(webmailUrl: string, code: string): Promi
 
 const activeRefreshes = new Map<string, Promise<OAuthTokens>>();
 
+// Stalwart 1.0 rotates refresh tokens, and a superseded one presented after
+// its successor was used revokes the whole grant (access token 401, current
+// refresh token invalid_grant; checked on 1.0.0). Several holders refresh the
+// same account from their own copy of the bundle: the live client, the
+// unified inbox, the background push task. One that still holds the old
+// token after another rotated it gets the successor here instead of spending
+// the dead token and signing the account out.
+const rotatedTo = new Map<string, OAuthTokens>();
+const ROTATED_MAX = 64;
+// A successor whose access token has less left than this is refreshed in
+// turn rather than handed out.
+const SUCCESSOR_MIN_LIFETIME_MS = 60_000;
+
+function rememberRotation(previous: string, next: OAuthTokens): void {
+  if (!next.refreshToken || next.refreshToken === previous) return;
+  if (!rotatedTo.has(previous) && rotatedTo.size >= ROTATED_MAX) {
+    const oldest = rotatedTo.keys().next().value;
+    if (oldest !== undefined) rotatedTo.delete(oldest);
+  }
+  rotatedTo.set(previous, next);
+}
+
+function latestRotation(refreshToken: string): OAuthTokens | null {
+  let latest: OAuthTokens | null = null;
+  let key = refreshToken;
+  for (let hop = 0; hop < ROTATED_MAX; hop++) {
+    const next = rotatedTo.get(key);
+    if (!next?.refreshToken) break;
+    latest = next;
+    key = next.refreshToken;
+  }
+  return latest;
+}
+
+/** Test hook. */
+export function resetRefreshRotations(): void {
+  rotatedTo.clear();
+}
+
 // OAuth refresh — exchanges the refresh token at the original token endpoint
 // for a new access token. Returns the updated bundle so the caller can
 // persist it.
@@ -543,6 +582,14 @@ export async function refreshOAuthAccessToken(tokens: OAuthTokens): Promise<OAut
     throw new HandoffError('No refresh token available');
   }
 
+  const successor = latestRotation(tokens.refreshToken);
+  if (successor) {
+    if (successor.expiresAt == null || successor.expiresAt - Date.now() > SUCCESSOR_MIN_LIFETIME_MS) {
+      return { ...successor, source: tokens.source };
+    }
+    return refreshOAuthAccessToken({ ...successor, source: tokens.source });
+  }
+
   const cacheKey = tokens.refreshToken;
   let promise = activeRefreshes.get(cacheKey);
 
@@ -599,7 +646,7 @@ export async function refreshOAuthAccessToken(tokens: OAuthTokens): Promise<OAut
         if (!data.access_token) {
           throw new HandoffError('Token refresh response missing access_token');
         }
-        return {
+        const next: OAuthTokens = {
           accessToken: data.access_token,
           refreshToken: data.refresh_token ?? tokens.refreshToken!,
           expiresAt: data.expires_in ? Date.now() + data.expires_in * 1000 : undefined,
@@ -607,6 +654,8 @@ export async function refreshOAuthAccessToken(tokens: OAuthTokens): Promise<OAut
           clientId: tokens.clientId,
           source: tokens.source,
         };
+        rememberRotation(cacheKey, next);
+        return next;
       } finally {
         activeRefreshes.delete(cacheKey);
       }