diff --git a/app/api/account/stalwart/jmap/route.ts b/app/api/account/stalwart/jmap/route.ts
index 150e5e2a..76c9a37d 100644
--- a/app/api/account/stalwart/jmap/route.ts
+++ b/app/api/account/stalwart/jmap/route.ts
@@ -55,9 +55,23 @@ export async function POST(request: NextRequest) {
       logger.error('Stalwart JMAP passthrough redirect error', { error: error.message });
       return NextResponse.json({ error: error.message }, { status: 502 });
     }
+    // `fetch failed` from undici is too generic to debug — the real reason
+    // (ENOTFOUND, ECONNREFUSED, self-signed TLS, …) lives on `error.cause`.
+    const err = error as Error & { cause?: { code?: string; message?: string } };
     logger.error('Stalwart JMAP passthrough error', {
-      error: error instanceof Error ? error.message : 'Unknown',
+      error: err?.message ?? 'Unknown',
+      causeCode: err?.cause?.code,
+      causeMessage: err?.cause?.message,
     });
+    // The server this process failed to reach is the user's own mail server,
+    // so the reason is worth surfacing: an opaque 500 leaves operators with
+    // nothing to act on.
+    if (err?.cause?.code) {
+      return NextResponse.json(
+        { error: `Cannot reach the JMAP server (${err.cause.code})` },
+        { status: 502 },
+      );
+    }
     return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
   }
 }
diff --git a/package-lock.json b/package-lock.json
index bce5d6b7..5b47f334 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -6280,6 +6280,7 @@
       "version": "2.3.2",
       "resolved": "https://registry.npmjs.org/fsevents/-/fsevents-2.3.2.tgz",
       "integrity": "sha512-xiqMQR4xAeHTuB9uWm+fFRcIOgKBMiOBP+eXiyT7jsgVCq1bkVygt00oASowB7EdtpOHaaPgKt812P9ab+DDKA==",
+      "dev": true,
       "hasInstallScript": true,
       "license": "MIT",
       "optional": true,
@@ -7976,6 +7977,17 @@
         }
       }
     },
+    "node_modules/next-intl/node_modules/@swc/helpers": {
+      "version": "0.5.23",
+      "resolved": "https://registry.npmjs.org/@swc/helpers/-/helpers-0.5.23.tgz",
+      "integrity": "sha512-5lSsMOTXURePglDfvuAQUqkGek9Hg2kksOYay2m0+XR++b2NWYL/4sWyuvVBIs8oKnJaxkdi9whaL/sqN13afw==",
+      "license": "Apache-2.0",
+      "optional": true,
+      "peer": true,
+      "dependencies": {
+        "tslib": "^2.8.0"
+      }
+    },
     "node_modules/next/node_modules/postcss": {
       "version": "8.4.31",
       "resolved": "https://registry.npmjs.org/postcss/-/postcss-8.4.31.tgz",