First try to install Stalwart as a mail software

nixos/modules/mail.nix

@@ -7,8 +7,6 @@
 *   and use your personal admin account or create one using the fallback admin password.
 * Create users with mail boxes: Go to the admin interface and create them.
 * Stalwart mailserver docs can be found at https://stalw.art/docs
-* DNS-Records: Collect the right DNS entries from the management interface and copy them to the DNS hoster. Caution:
-*   Not all entries are applicable since we relay via HRZ.
 */
 {
   config,