Restrict BuddyPress Email Domains at Registration
BuddyPress Restrict Email Domains is a handy new plugin that performs a check during the BuddyPress registration process. This plugin offers more control over email domains that are allowed to register on your site and gives you the option to restrict certain email providers. Currently it only works with BuddyPress on single user WordPress installations. However, if you want to use it with Multisite, there is a simple filter you can add to enable it with Multisite.