WPMU DEV MASS EMAIL SENDER Plugin
Send emails to every user on a Multisite network and allow users to opt in and out.
WPMU DEV MASS EMAIL SENDER Plugin allows you to send an email to every user on your network via a simple form.
If you have new features, site maintenance or just want to get in touch with your users…this is for you.
Plus, users have the option to unsubscribe from the mailing list any time.
Please visit official plugin page for more information