diff --git a/app/api/auth/session/route.ts b/app/api/auth/session/route.ts
index 0fbd07f7..2ec20847 100644
--- a/app/api/auth/session/route.ts
+++ b/app/api/auth/session/route.ts
@@ -10,6 +10,7 @@ import {
   setStalwartAuthContextInStore,
 } from '@/lib/stalwart/auth-context';
 import { configManager } from '@/lib/admin/config-manager';
+import { isPublicHttpUrl } from '@/lib/security/url-guard';
 import { recordLogin } from '@/lib/telemetry/login-tracker';
 
 const COOKIE_OPTIONS = {
@@ -38,10 +39,37 @@ export async function POST(request: NextRequest) {
       return NextResponse.json({ error: 'Missing required fields' }, { status: 400 });
     }
 
+    // Pin the upstream URL to the configured JMAP server so an unauthenticated
+    // caller cannot point this route at internal hosts. Only when no server URL
+    // is configured AND the deployment explicitly allows custom JMAP endpoints
+    // do we honor the body URL — and even then it must be a public URL.
+    await configManager.ensureLoaded();
+    const configuredServerUrl =
+      configManager.get<string>('jmapServerUrl', '') ||
+      process.env.JMAP_SERVER_URL ||
+      process.env.NEXT_PUBLIC_JMAP_SERVER_URL ||
+      '';
+    const allowCustomEndpoint = configManager.get<boolean>('allowCustomJmapEndpoint', false);
+
+    let upstreamUrl: string;
+    let upstreamTrusted: boolean;
+    if (configuredServerUrl) {
+      upstreamUrl = configuredServerUrl;
+      upstreamTrusted = true;
+    } else if (allowCustomEndpoint) {
+      if (!(await isPublicHttpUrl(serverUrl))) {
+        return NextResponse.json({ error: 'Server URL is not allowed' }, { status: 400 });
+      }
+      upstreamUrl = serverUrl;
+      upstreamTrusted = false;
+    } else {
+      return NextResponse.json({ error: 'JMAP server not configured' }, { status: 500 });
+    }
+
     const slot = typeof bodySlot === 'number' && bodySlot >= 0 && bodySlot <= 4 ? bodySlot : getSlot(request);
     const cookieName = sessionCookieName(slot);
     const authHeader = `Basic ${Buffer.from(`${username}:${password}`).toString('base64')}`;
-    const normalizedServerUrl = await verifyJmapAuth(serverUrl, authHeader);
+    const normalizedServerUrl = await verifyJmapAuth(upstreamUrl, authHeader, { trusted: upstreamTrusted });
     const token = encryptSession(normalizedServerUrl, username, password);
     const cookieStore = await cookies();
     cookieStore.set(cookieName, token, COOKIE_OPTIONS);
diff --git a/app/api/auth/stalwart-context/route.ts b/app/api/auth/stalwart-context/route.ts
index 138cacc2..7220673e 100644
--- a/app/api/auth/stalwart-context/route.ts
+++ b/app/api/auth/stalwart-context/route.ts
@@ -2,6 +2,8 @@ import { NextRequest, NextResponse } from 'next/server';
 import { logger } from '@/lib/logger';
 import { JmapAuthVerificationError, verifyJmapAuth } from '@/lib/auth/verify-jmap-auth';
 import { setStalwartAuthContext } from '@/lib/stalwart/auth-context';
+import { configManager } from '@/lib/admin/config-manager';
+import { isPublicHttpUrl } from '@/lib/security/url-guard';
 import { recordLogin } from '@/lib/telemetry/login-tracker';
 
 function getSlot(request: NextRequest, bodySlot: unknown): number {
@@ -24,8 +26,35 @@ export async function POST(request: NextRequest) {
       return NextResponse.json({ error: 'Missing required fields' }, { status: 400 });
     }
 
+    // Pin the upstream URL to the configured JMAP server so an unauthenticated
+    // caller cannot point this route at internal hosts. Only when no server URL
+    // is configured AND the deployment explicitly allows custom JMAP endpoints
+    // do we honor the body URL — and even then it must be a public URL.
+    await configManager.ensureLoaded();
+    const configuredServerUrl =
+      configManager.get<string>('jmapServerUrl', '') ||
+      process.env.JMAP_SERVER_URL ||
+      process.env.NEXT_PUBLIC_JMAP_SERVER_URL ||
+      '';
+    const allowCustomEndpoint = configManager.get<boolean>('allowCustomJmapEndpoint', false);
+
+    let upstreamUrl: string;
+    let upstreamTrusted: boolean;
+    if (configuredServerUrl) {
+      upstreamUrl = configuredServerUrl;
+      upstreamTrusted = true;
+    } else if (allowCustomEndpoint) {
+      if (!(await isPublicHttpUrl(serverUrl))) {
+        return NextResponse.json({ error: 'Server URL is not allowed' }, { status: 400 });
+      }
+      upstreamUrl = serverUrl;
+      upstreamTrusted = false;
+    } else {
+      return NextResponse.json({ error: 'JMAP server not configured' }, { status: 500 });
+    }
+
     const slot = getSlot(request, bodySlot);
-    const normalizedServerUrl = await verifyJmapAuth(serverUrl, authHeader);
+    const normalizedServerUrl = await verifyJmapAuth(upstreamUrl, authHeader, { trusted: upstreamTrusted });
 
     await setStalwartAuthContext(slot, {
       serverUrl: normalizedServerUrl,
diff --git a/lib/__tests__/verify-jmap-auth.test.ts b/lib/__tests__/verify-jmap-auth.test.ts
index 1ccce302..efc8dfe8 100644
--- a/lib/__tests__/verify-jmap-auth.test.ts
+++ b/lib/__tests__/verify-jmap-auth.test.ts
@@ -138,4 +138,38 @@ describe('verifyJmapAuth SSRF protection', () => {
     });
     expect(fetchSpy).not.toHaveBeenCalled();
   });
+
+  it('with trusted=true, accepts a hostname resolving to a private IP', async () => {
+    lookup.mockResolvedValue([{ address: '10.0.20.5', family: 4 }]);
+    fetchSpy.mockResolvedValueOnce(
+      new Response(JSON.stringify({ apiUrl: 'https://mail.internal/api', accounts: {} }), {
+        status: 200,
+        headers: { 'content-type': 'application/json' },
+      }),
+    );
+    const { verifyJmapAuth } = await load();
+    await expect(
+      verifyJmapAuth('https://mail.internal', 'Bearer x', { trusted: true }),
+    ).resolves.toBe('https://mail.internal');
+    expect(fetchSpy).toHaveBeenCalledWith(
+      'https://mail.internal/.well-known/jmap',
+      expect.objectContaining({ redirect: 'manual' }),
+    );
+  });
+
+  it('with trusted=true, still rejects unsupported protocols', async () => {
+    const { verifyJmapAuth } = await load();
+    await expect(
+      verifyJmapAuth('file:///etc/passwd', 'Bearer x', { trusted: true }),
+    ).rejects.toMatchObject({ status: 400 });
+    expect(fetchSpy).not.toHaveBeenCalled();
+  });
+
+  it('with trusted=true, still rejects an invalid Authorization header', async () => {
+    const { verifyJmapAuth } = await load();
+    await expect(
+      verifyJmapAuth('https://mail.internal', 'NotAuth', { trusted: true }),
+    ).rejects.toMatchObject({ status: 400 });
+    expect(fetchSpy).not.toHaveBeenCalled();
+  });
 });
diff --git a/lib/auth/verify-jmap-auth.ts b/lib/auth/verify-jmap-auth.ts
index ba421e0c..1d2d1616 100644
--- a/lib/auth/verify-jmap-auth.ts
+++ b/lib/auth/verify-jmap-auth.ts
@@ -40,11 +40,15 @@ export function validateProxyAuthHeader(authHeader: string): void {
   }
 }
 
-export async function verifyJmapAuth(serverUrl: string, authHeader: string): Promise<string> {
+export async function verifyJmapAuth(
+  serverUrl: string,
+  authHeader: string,
+  options: { trusted?: boolean } = {},
+): Promise<string> {
   const normalizedServerUrl = normalizeJmapServerUrl(serverUrl);
   validateProxyAuthHeader(authHeader);
 
-  if (!(await isPublicHttpUrl(normalizedServerUrl))) {
+  if (!options.trusted && !(await isPublicHttpUrl(normalizedServerUrl))) {
     throw new JmapAuthVerificationError('Server URL is not allowed', 400);
   }
 
@@ -56,7 +60,7 @@ export async function verifyJmapAuth(serverUrl: string, authHeader: string): Pro
     let response: Response | undefined;
 
     for (let i = 0; i <= MAX_REDIRECTS; i++) {
-      if (!(await isPublicHttpUrl(currentUrl))) {
+      if (!options.trusted && !(await isPublicHttpUrl(currentUrl))) {
         throw new JmapAuthVerificationError('Server URL is not allowed', 400);
       }