diff --git a/core/admin/mailu/internal/views/auth.py b/core/admin/mailu/internal/views/auth.py
index 2baeddce..9c3e6a85 100644
--- a/core/admin/mailu/internal/views/auth.py
+++ b/core/admin/mailu/internal/views/auth.py
@@ -50,7 +50,7 @@ def user_authentication():
     if (not flask_login.current_user.is_anonymous
         and flask_login.current_user.enabled):
         response = flask.Response()
-        response.headers["X-User"] = flask_login.current_user.get_id()
+        response.headers["X-User"] = models.IdnaEmail.process_bind_param(flask_login, flask_login.current_user.get_id(), "")
         response.headers["X-User-Token"] = models.User.get_temp_token(flask_login.current_user.get_id())
         return response
     return flask.abort(403)