Hack around sieve execution for multiple recipients.

nixos/modules/mailman.nix

@@ -43,7 +43,10 @@ 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.
+        };
       };
     };