Thanks for plugin.
Have this for test on local install multi-network and then on a live install
Warning: Cannot modify header information - headers already sent by (output started at /Users/valuser/Documents/Websites/mydomain.dev/wp-content/plugins/mass-messaging-in-buddypress/mass-messaging.php:468) in /Users/valuser/Documents/Websites/mydomain.dev/wp-includes/pluggable.php on line 881
no errors so far though limited tests so far.
It may not be anything to worry about. If it is i'd be thrilled to know.