Hack around sieve execution for multiple recipients.

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'";
           };