If you manage several websites and need a notification system that operates outside of WordPress, we created our excellent product as a stand-alone solution. It uses Node JS and PHP multi-threading, which is 15x quicker than a WordPress plugin and uses very little server resources. It also supports all plugin functionality. We strongly advise using this solution if you have more than 100K subscribers, since once you hit this milestone, WordPress begins to consume all server resources due to excessive traffic.
Telegram, Facebook Messenger, Facebook alerts, newsletters, SMS, Viber, iOS, Android, Windows Phone 8, BlackBerry 10, Chrome, Safari, Edge, Opera, Samsung browser, and Firefox all have full notification systems built in. Now check!