Actually reject mails sent to system accounts and add respective testcase

mailsystem/default.nix

@@ -112,7 +112,17 @@ in {
               account will be rejected.
             '';
           };
+
+          rejectMessage = lib.mkOption {
+            type = lib.types.str;
+            default = "This account cannot receive emails.";
+            description = ''
+              The message that will be returned to the sender when an email is
+              sent to a system account.
+            '';
+          };
         };
+
         config.name = lib.mkDefault name;
       }));
       example = {