diff --git a/core/admin/mailu/internal/nginx.py b/core/admin/mailu/internal/nginx.py
index 02998fed..03142e62 100644
--- a/core/admin/mailu/internal/nginx.py
+++ b/core/admin/mailu/internal/nginx.py
@@ -93,10 +93,9 @@ def handle_authentication(headers):
             app.logger.warn(f'Received undecodable user/password from nginx: {raw_user_email!r}/{raw_password!r}')
         else:
             try:
-                user = models.User.query.get(user_email)
-                is_valid_user = True
-            except ValueError:
-                pass
+                if '@' in user_email:
+                    user = models.User.query.get(user_email)
+                    is_valid_user = True
             except sqlalchemy.exc.StatementError as exc:
                 exc = str(exc).split('\n', 1)[0]
                 app.logger.warn(f'Invalid user {user_email!r}: {exc}')