diff --git a/app/(main)/admin/_tabs/auth.tsx b/app/(main)/admin/_tabs/auth.tsx
index 4387ef2e..5bda49b3 100644
--- a/app/(main)/admin/_tabs/auth.tsx
+++ b/app/(main)/admin/_tabs/auth.tsx
@@ -273,6 +273,7 @@ export function AuthTab() {
         <Text label="OAuth Client ID" configKey="oauthClientId" value={currentValue('oauthClientId') as string} source={config.oauthClientId?.source} onChange={handleChange} onRevert={handleRevert} />
         <Text label="OAuth Client Secret" configKey="oauthClientSecret" value={currentValue('oauthClientSecret') as string} source={config.oauthClientSecret?.source} onChange={handleChange} onRevert={handleRevert} type="password" placeholder={config.oauthClientSecret?.hasValue ? '••••••••  (saved - type to replace)' : undefined} />
         <Text label="OAuth Issuer URL" configKey="oauthIssuerUrl" value={currentValue('oauthIssuerUrl') as string} source={config.oauthIssuerUrl?.source} onChange={handleChange} onRevert={handleRevert} placeholder="https://auth.example.com" />
+        <Toggle label="Allow private OAuth endpoints" description="Permit discovery to resolve to RFC-1918 / loopback hosts. Enable only for split-DNS deployments where the mail server's public hostname resolves to an internal IP." configKey="oauthAllowPrivateEndpoints" value={currentValue('oauthAllowPrivateEndpoints') as boolean} source={config.oauthAllowPrivateEndpoints?.source} onChange={handleChange} onRevert={handleRevert} />
         <Text label="OAuth Scopes" description="Space-separated scopes that replace the defaults. Leave blank to use the built-in scope list." configKey="oauthScopes" value={currentValue('oauthScopes') as string} source={config.oauthScopes?.source} onChange={handleChange} onRevert={handleRevert} placeholder="openid email offline_access" />
         <Text label="OAuth Extra Scopes" description="Additional space-separated scopes appended to the defaults." configKey="oauthExtraScopes" value={currentValue('oauthExtraScopes') as string} source={config.oauthExtraScopes?.source} onChange={handleChange} onRevert={handleRevert} placeholder="urn:ietf:params:oauth:..." />
       </Section>
diff --git a/app/api/auth/sso/start/route.ts b/app/api/auth/sso/start/route.ts
index fd0e861f..9edfbd31 100644
--- a/app/api/auth/sso/start/route.ts
+++ b/app/api/auth/sso/start/route.ts
@@ -3,9 +3,8 @@ import { cookies } from 'next/headers';
 import { logger } from '@/lib/logger';
 import { encryptPayload } from '@/lib/auth/crypto';
 import { generateCodeVerifierServer, generateCodeChallengeServer, generateStateServer } from '@/lib/oauth/pkce-server';
-import { getRequiredConfig } from '@/lib/oauth/token-exchange';
+import { getRequiredConfig, getDiscoveryValidator } from '@/lib/oauth/token-exchange';
 import { discoverOAuth } from '@/lib/oauth/discovery';
-import { isPublicHttpUrl } from '@/lib/security/url-guard';
 import { getOauthScopes } from '@/lib/oauth/tokens';
 import { getCookieOptions } from '@/lib/oauth/cookie-config';
 import { hasSessionSecret } from '@/lib/auth/session-secret';
@@ -62,7 +61,7 @@ export async function POST(request: NextRequest) {
     }
 
     const { clientId, discoveryUrl } = getRequiredConfig(serverId);
-    const metadata = await discoverOAuth(discoveryUrl, { validateEndpoint: isPublicHttpUrl });
+    const metadata = await discoverOAuth(discoveryUrl, { validateEndpoint: getDiscoveryValidator() });
 
     if (!metadata?.authorization_endpoint) {
       return NextResponse.json({ error: 'OAuth discovery failed' }, { status: 502 });
diff --git a/app/api/auth/totp-token-exchange/route.ts b/app/api/auth/totp-token-exchange/route.ts
index e19bc781..11fa1a6b 100644
--- a/app/api/auth/totp-token-exchange/route.ts
+++ b/app/api/auth/totp-token-exchange/route.ts
@@ -2,6 +2,7 @@ import { NextRequest, NextResponse } from 'next/server';
 import { cookies } from 'next/headers';
 import { logger } from '@/lib/logger';
 import { discoverOAuth } from '@/lib/oauth/discovery';
+import { getDiscoveryValidator } from '@/lib/oauth/token-exchange';
 import { refreshTokenCookieName, refreshTokenServerCookieName } from '@/lib/oauth/tokens';
 import { getCookieOptions } from '@/lib/oauth/cookie-config';
 import { readFileEnv } from '@/lib/read-file-env';
@@ -52,9 +53,13 @@ async function tryTokenRequest(
   }
 }
 
-async function findTokenEndpoint(serverUrl: string): Promise<string | null> {
+async function findTokenEndpoint(serverUrl: string, adminTrusted: boolean): Promise<string | null> {
+  // Admin-trusted callers (matched server entry or configured JMAP server URL)
+  // honor the `oauthAllowPrivateEndpoints` opt-in. User-supplied URLs always
+  // go through the SSRF validator regardless of the setting.
+  const validateEndpoint = adminTrusted ? getDiscoveryValidator() : isPublicHttpUrl;
   // 1. Try OAuth discovery
-  const metadata = await discoverOAuth(serverUrl, { validateEndpoint: isPublicHttpUrl });
+  const metadata = await discoverOAuth(serverUrl, { validateEndpoint });
   if (metadata?.token_endpoint) return metadata.token_endpoint;
 
   // 2. Try common Stalwart token endpoint paths directly
@@ -105,14 +110,17 @@ export async function POST(request: NextRequest) {
 
     let upstreamUrl: string;
     let resolvedServerId: string | null = null;
+    let adminTrusted = false;
     const requestedEntry = findServerById(serverList, requestedServerId);
     const matchedEntry = requestedEntry || findServerByUrl(serverList, serverUrl);
 
     if (matchedEntry) {
       upstreamUrl = matchedEntry.url;
       resolvedServerId = matchedEntry.id;
+      adminTrusted = true;
     } else if (configuredServerUrl) {
       upstreamUrl = configuredServerUrl;
+      adminTrusted = true;
     } else if (allowCustomEndpoint) {
       if (!(await isPublicHttpUrl(serverUrl))) {
         logger.warn('TOTP token exchange: rejected non-public server URL');
@@ -123,7 +131,7 @@ export async function POST(request: NextRequest) {
       return NextResponse.json({ error: 'jmap_server_not_configured' }, { status: 500 });
     }
 
-    const tokenEndpoint = await findTokenEndpoint(upstreamUrl);
+    const tokenEndpoint = await findTokenEndpoint(upstreamUrl, adminTrusted);
     if (!tokenEndpoint) {
       logger.warn('TOTP token exchange: no token endpoint found');
       return NextResponse.json({ error: 'no_token_endpoint', detail: 'Could not discover OAuth token endpoint on the mail server' }, { status: 404 });
diff --git a/lib/__tests__/oauth-discovery.test.ts b/lib/__tests__/oauth-discovery.test.ts
index 609a9294..5b8228af 100644
--- a/lib/__tests__/oauth-discovery.test.ts
+++ b/lib/__tests__/oauth-discovery.test.ts
@@ -146,6 +146,23 @@ describe('oauth/discovery', () => {
     expect(consoleSpy).toHaveBeenCalled();
   });
 
+  it('accepts private/loopback endpoints when validateEndpoint is omitted (admin opted in)', async () => {
+    // Split-DNS deployments: mail.example.com resolves to an RFC-1918 address
+    // locally. With the SSRF validator off, discovery must succeed.
+    vi.stubGlobal('fetch', vi.fn().mockResolvedValueOnce({
+      ok: true,
+      json: () => Promise.resolve({
+        issuer: 'https://mail.example.com',
+        authorization_endpoint: 'http://10.0.0.5/authorize',
+        token_endpoint: 'http://10.0.0.5/token',
+      }),
+    }));
+
+    const result = await discoverOAuth('https://mail.example.com');
+
+    expect(result?.token_endpoint).toBe('http://10.0.0.5/token');
+  });
+
   it('caches results - second call for same server URL does not re-fetch', async () => {
     vi.stubGlobal('fetch', vi.fn().mockResolvedValueOnce({
       ok: true,
diff --git a/lib/admin/types.ts b/lib/admin/types.ts
index 320c783b..57e8535b 100644
--- a/lib/admin/types.ts
+++ b/lib/admin/types.ts
@@ -153,6 +153,7 @@ export const CONFIG_ENV_MAP: Record<string, { envVar: string; fileEnvVar?: strin
   oauthIssuerUrl: { envVar: 'OAUTH_ISSUER_URL', type: 'url', defaultValue: '' },
   oauthScopes: { envVar: 'OAUTH_SCOPES', type: 'string', defaultValue: '' },
   oauthExtraScopes: { envVar: 'OAUTH_EXTRA_SCOPES', type: 'string', defaultValue: '' },
+  oauthAllowPrivateEndpoints: { envVar: 'OAUTH_ALLOW_PRIVATE_ENDPOINTS', type: 'boolean', defaultValue: false },
   allowCustomJmapEndpoint: { envVar: 'ALLOW_CUSTOM_JMAP_ENDPOINT', type: 'boolean', defaultValue: false },
   jmapServers: { envVar: 'JMAP_SERVERS', type: 'json', defaultValue: [] },
   jmapServerAutoPickByDomain: { envVar: 'JMAP_SERVER_AUTO_PICK_BY_DOMAIN', type: 'boolean', defaultValue: false },
diff --git a/lib/oauth/token-exchange.ts b/lib/oauth/token-exchange.ts
index ea275fdd..32f1956c 100644
--- a/lib/oauth/token-exchange.ts
+++ b/lib/oauth/token-exchange.ts
@@ -1,11 +1,21 @@
 import { logger } from '@/lib/logger';
 import { discoverOAuth } from '@/lib/oauth/discovery';
-import type { OAuthMetadata } from '@/lib/oauth/discovery';
+import type { EndpointValidator, OAuthMetadata } from '@/lib/oauth/discovery';
 import { isPublicHttpUrl } from '@/lib/security/url-guard';
 import { readFileEnv } from '@/lib/read-file-env';
 import { configManager } from '@/lib/admin/config-manager';
 import { parseJmapServers, findServerById } from '@/lib/admin/jmap-servers';
 
+// SSRF guard for OAuth discovery. When `oauthAllowPrivateEndpoints` is set,
+// the admin opts in to discovery resolving to RFC-1918 / loopback hosts —
+// required for split-DNS deployments where the JMAP server's public hostname
+// resolves to an internal IP locally. The guard remains in force for any
+// caller that passes a user-supplied serverUrl (see totp-token-exchange).
+export function getDiscoveryValidator(): EndpointValidator | undefined {
+  const allowPrivate = configManager.get<boolean>('oauthAllowPrivateEndpoints', false);
+  return allowPrivate ? undefined : isPublicHttpUrl;
+}
+
 function getGlobalClientSecret(): string {
   const adminSecret = configManager.get<string>('oauthClientSecret', '');
   if (adminSecret) return adminSecret;
@@ -47,7 +57,7 @@ function getClientSecret(serverId?: string | null): string {
 
 export async function getTokenEndpoint(serverId?: string | null): Promise<string> {
   const { discoveryUrl } = getRequiredConfig(serverId);
-  const metadata = await discoverOAuth(discoveryUrl, { validateEndpoint: isPublicHttpUrl });
+  const metadata = await discoverOAuth(discoveryUrl, { validateEndpoint: getDiscoveryValidator() });
   if (!metadata?.token_endpoint) {
     throw new Error('OAuth token endpoint not found');
   }
@@ -56,7 +66,7 @@ export async function getTokenEndpoint(serverId?: string | null): Promise<string
 
 export async function getMetadata(serverId?: string | null): Promise<OAuthMetadata | null> {
   const { discoveryUrl } = getRequiredConfig(serverId);
-  return discoverOAuth(discoveryUrl, { validateEndpoint: isPublicHttpUrl });
+  return discoverOAuth(discoveryUrl, { validateEndpoint: getDiscoveryValidator() });
 }
 
 export function buildOAuthParams(base: Record<string, string>, serverId?: string | null): URLSearchParams {