diff --git a/app/(main)/admin/_pages/auth.tsx b/app/(main)/admin/_pages/auth.tsx
index 1d1349092..04afb2e38 100644
--- a/app/(main)/admin/_pages/auth.tsx
+++ b/app/(main)/admin/_pages/auth.tsx
@@ -34,12 +34,25 @@ export function AuthPage() {
   const setMessage = useAdminFeedback();
   const [configReadOnly, setConfigReadOnly] = useState(false);
 
+  // The setup runs as the mail account signed in to this webmail. Asked again
+  // on focus, since the admin signs in from another tab and comes back.
+  const [setupSignIn, setSetupSignIn] = useState<{ signedIn: boolean; username?: string } | null>(null);
+
   useEffect(() => {
     fetchConfig();
     apiFetch('/api/setup/status')
       .then(res => (res.ok ? res.json() : null))
       .then(data => setConfigReadOnly(data?.readOnly === true))
       .catch(() => {});
+    const checkSignIn = () => {
+      apiFetch('/api/admin/oauth/setup')
+        .then(res => (res.ok ? res.json() : null))
+        .then(data => { if (data) setSetupSignIn(data); })
+        .catch(() => {});
+    };
+    checkSignIn();
+    window.addEventListener('focus', checkSignIn);
+    return () => window.removeEventListener('focus', checkSignIn);
   }, []);
 
   async function fetchConfig() {
@@ -260,7 +273,7 @@ export function AuthPage() {
             description="Registers an OAuth client on the connected Stalwart server, generates a client secret, and saves the settings here. Requires your Stalwart account to have admin permissions."
           >
             <div className="flex flex-col items-start gap-1 sm:items-end">
-              <Button type="button" size="sm" className="gap-2" onClick={openSetupDialog} disabled={setupRunning || configReadOnly}>
+              <Button type="button" size="sm" className="gap-2" onClick={openSetupDialog} disabled={setupRunning || configReadOnly || setupSignIn?.signedIn === false}>
                 {setupRunning ? <Loader2 className="w-4 h-4 animate-spin" /> : <Sparkles className="w-4 h-4" />}
                 {setupRunning ? 'Configuring…' : 'Set up automagically'}
               </Button>
@@ -269,6 +282,16 @@ export function AuthPage() {
                   Not available while the configuration is read-only (ADMIN_CONFIG_READONLY).
                 </p>
               )}
+              {!configReadOnly && setupSignIn?.signedIn === false && (
+                <p className="max-w-xs text-xs text-muted-foreground sm:text-end">
+                  Only works while you are signed in to{' '}
+                  <a href="/" target="_blank" rel="noopener" className="text-primary hover:underline">this webmail</a>
+                  {' '}with a Stalwart admin account in this browser. A login to the Stalwart web admin does not count.
+                </p>
+              )}
+              {!configReadOnly && setupSignIn?.signedIn && setupSignIn.username && (
+                <p className="text-xs text-muted-foreground">Runs as {setupSignIn.username}</p>
+              )}
             </div>
           </SettingItem>
           <ConfigToggleItem label="Enabled" configKey="oauthEnabled" value={currentValue('oauthEnabled') as boolean} source={config.oauthEnabled?.source} onChange={handleChange} onRevert={handleRevert} />
diff --git a/app/api/admin/oauth/setup/__tests__/route.test.ts b/app/api/admin/oauth/setup/__tests__/route.test.ts
index 7046502d1..c3a416fb1 100644
--- a/app/api/admin/oauth/setup/__tests__/route.test.ts
+++ b/app/api/admin/oauth/setup/__tests__/route.test.ts
@@ -7,6 +7,7 @@ import { NextRequest } from 'next/server';
 let readOnly = false;
 let saveError: Error | null = null;
 let stalwart016 = false;
+let signedIn = true;
 const jmapCalls: Array<[string, Record<string, unknown>]> = [];
 
 vi.mock('@/lib/admin/session', () => ({
@@ -14,12 +15,14 @@ vi.mock('@/lib/admin/session', () => ({
   getClientIP: () => '127.0.0.1',
 }));
 vi.mock('@/lib/stalwart/credentials', () => ({
-  getStalwartCredentials: async () => ({
-    serverUrl: 'https://mail.example.org',
-    authHeader: 'Basic x',
-    trusted: true,
-    username: 'admin@example.org',
-  }),
+  getStalwartCredentials: async () => (signedIn
+    ? {
+      serverUrl: 'https://mail.example.org',
+      authHeader: 'Basic x',
+      trusted: true,
+      username: 'admin@example.org',
+    }
+    : null),
 }));
 vi.mock('@/lib/admin/paths', async (importOriginal) => ({
   ...(await importOriginal<typeof import('@/lib/admin/paths')>()),
@@ -49,7 +52,7 @@ vi.mock('@/lib/stalwart/server-fetch', () => ({
   },
 }));
 
-import { POST } from '../route';
+import { GET, POST } from '../route';
 
 function setup() {
   return POST(new NextRequest('http://localhost/api/admin/oauth/setup', {
@@ -62,6 +65,7 @@ beforeEach(() => {
   readOnly = false;
   saveError = null;
   stalwart016 = false;
+  signedIn = true;
   jmapCalls.length = 0;
 });
 
@@ -89,6 +93,14 @@ describe('POST /api/admin/oauth/setup', () => {
     expect(create).not.toHaveProperty('firstParty');
   });
 
+  it('tells the admin to sign in to the webmail when no mail account is signed in here', async () => {
+    signedIn = false;
+    const res = await setup();
+    expect(res.status).toBe(400);
+    expect((await res.json()).error).toMatch(/Sign in to this webmail.*Stalwart web admin does not count/);
+    expect(jmapCalls).toEqual([]);
+  });
+
   it('refuses in read-only mode without sending anything to Stalwart (#1250)', async () => {
     readOnly = true;
     const res = await setup();
@@ -104,3 +116,23 @@ describe('POST /api/admin/oauth/setup', () => {
     expect(jmapCalls.at(-1)).toEqual(['x:OAuthClient/set', { accountId: 'a', destroy: ['h'] }]);
   });
 });
+
+describe('GET /api/admin/oauth/setup', () => {
+  const check = () => GET(new NextRequest('http://localhost/api/admin/oauth/setup'));
+
+  it('names the mail account the setup would run as', async () => {
+    const res = await check();
+    expect(await res.json()).toEqual({ signedIn: true, username: 'admin@example.org' });
+  });
+
+  it('says so when no mail account is signed in to this webmail', async () => {
+    signedIn = false;
+    const res = await check();
+    expect(await res.json()).toEqual({ signedIn: false });
+  });
+
+  it('never hands the credential to the page', async () => {
+    const body = JSON.stringify(await (await check()).json());
+    expect(body).not.toContain('Basic');
+  });
+});
diff --git a/app/api/admin/oauth/setup/route.ts b/app/api/admin/oauth/setup/route.ts
index 6727366ad..074b5cfdf 100644
--- a/app/api/admin/oauth/setup/route.ts
+++ b/app/api/admin/oauth/setup/route.ts
@@ -102,6 +102,20 @@ function isValidOriginUrl(value: string): boolean {
   return /^https?:\/\/[^/]+$/.test(value);
 }
 
+// The setup acts as the mail account signed in to this webmail, not as the
+// admin: a login to the Stalwart web admin sets no cookie here.
+const NOT_SIGNED_IN =
+  'Sign in to this webmail with a Stalwart admin account in this browser, then retry. '
+  + 'A login to the Stalwart web admin does not count.';
+
+/** Which mail account the setup would run as, so the page can say before it starts. */
+export async function GET(request: NextRequest) {
+  const auth = await requireAdminAuth(request);
+  if ('error' in auth) return auth.error;
+  const creds = await getStalwartCredentials(request);
+  return NextResponse.json(creds ? { signedIn: true, username: creds.username } : { signedIn: false });
+}
+
 export async function POST(request: NextRequest) {
   try {
     const auth = await requireAdminAuth(request);
@@ -120,10 +134,7 @@ export async function POST(request: NextRequest) {
     const ip = getClientIP(request);
     const creds = await getStalwartCredentials(request);
     if (!creds) {
-      return NextResponse.json(
-        { error: 'No Stalwart session available. Sign in to your mail account in another tab and retry.' },
-        { status: 400 },
-      );
+      return NextResponse.json({ error: NOT_SIGNED_IN }, { status: 400 });
     }
 
     const body = await request.json() as SetupRequestBody;