The WordPress Mass Email Sender Plugin by WPMU DEV
This plugin helps to send emails to every user according to the requirement on a multisite network. It is easy to use and integrated with shortcodes. Users can unsubscribe to the mailing list anytime with just one click. It even has an option to send a test email, so that you can check your messages before sending to your entire network.

Bookly Waiting List (Add-on)
Maximize your bookings with our Waiting List add-on. Never miss out on potential appointments again by seamlessly managing your client’s availability. Save time and money with this essential tool.