mailserver: Disallow requesting DSN over SMTP

This still allows requesting a DSN over submission, so trusted clients
are not affected. It only affects sending DSN to other systems, which
now no longer takes place. This is done to avoid leaking rspamd
internals.

modules/mailserver/postfix.nix

@@ -141,6 +141,7 @@ lib.mkIf cfg.enable {
       # Postscreen
       smtpd = {
         type = "pass";
+        args = [ "-o" "smtpd_discard_ehlo_keywords=silent-discard,dsn" ];
       };
       smtp_inet = {
         # Partially overrides upstream