diff --git a/core/admin/mailu/internal/views/auth.py b/core/admin/mailu/internal/views/auth.py
index 9c3e6a85..1686e1cb 100644
--- a/core/admin/mailu/internal/views/auth.py
+++ b/core/admin/mailu/internal/views/auth.py
@@ -67,7 +67,7 @@ def basic_authentication():
         user = models.User.query.get(user_email.decode("utf8"))
         if nginx.check_credentials(user, password.decode('utf-8'), flask.request.remote_addr, "web"):
             response = flask.Response()
-            response.headers["X-User"] = user.email
+            response.headers["X-User"] = models.IdnaEmail.process_bind_param(flask_login, user.email, "")
             return response
     response = flask.Response(status=401)
     response.headers["WWW-Authenticate"] = 'Basic realm="Login Required"'
diff --git a/towncrier/newsfragments/1952.bugfix b/towncrier/newsfragments/1952.bugfix
new file mode 100644
index 00000000..655715f1
--- /dev/null
+++ b/towncrier/newsfragments/1952.bugfix
@@ -0,0 +1,3 @@
+Webmail and Radicale (webdav) were not useable with domains with special characters such as umlauts.
+Webmail and radicale now use punycode for logging in.
+Punycode was not used in the HTTP headers. This resulted in illegal non-ASCII HTTP headers.