Fix: Added missing callback.

server/services/monitoringService.js

@@ -278,6 +278,7 @@ angular.module('ffffng')
                                 Logger
                                     .tag('monitoring', 'mail-sending')
                                     .info('Monitoring disabled, skipping "%s" mail for: %s', name, mac);
+                                return mailCallback(null);
                             }
                         });
                     },