nixos/modules/mail.nix

@@ -184,6 +184,10 @@ in {
                 }
                 {"else" = false;}
               ];
+              # The sieve script only handles the last RCPT TO command (https://stalw.art/docs/sieve/variables).
+              # Since we want it to run for every recipient, we need to accept them one at a time. :-(
+              # This setting throws a temporary error for the second RCPT TO command after which the HRZ retries in a new connection.
+              max-recipients = 1;
             };
             data.script = "'redirects'";
           };

nixos/modules/mailman.nix

@@ -43,7 +43,11 @@ in {
         webHosts = [cfg.hostName];
         serve.enable = true; #
         # Don't include confirmation tokens in reply addresses, because we would need to send them to HRZ otherwise.
-        settings.mta.verp_confirmations = "no";
+        settings.mta = {
+          verp_confirmations = "no";
+          max_recipients = "1"; # We can only send to one recipient at a time due to how forwarding currently works. See also the mail module.
+          max_sessions_per_connection = "1";
+        };
       };
     };