Hack around sieve execution for multiple recipients.

nixos/modules/mailman.nix

@@ -45,7 +45,7 @@ in {
         # Don't include confirmation tokens in reply addresses, because we would need to send them to HRZ otherwise.
         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_recipients = 1; # We can only send to one recipient at a time due to how forwarding currently works. See also the mail module.
         };
       };
     };