Hack around sieve execution for multiple recipients.

nixos/modules/mailman.nix

@@ -46,6 +46,7 @@ in {
         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";
         };
       };
     };