BuddyPress.org

There actually is a function in buddypress but it is named different from the wpmu version, and I can see no way to administer it.

If you check your bp-core-signup.php file you will see:
`$limited_email_domains = get_site_option( ‘limited_email_domains’, ‘buddypress’ );`

Just change this line to:
`$limited_email_domains = get_site_option( ‘banned_email_domains’, ‘buddypress’ );`

This will solve signups from excluded domain list. Hopefully this will be incorporated in bpress core for compatability.