This BuddyPress plugin enables restriction of email domains during user registration for a single (non-multisite) WordPress installation of BuddyPress
Only works with BuddyPress on single user WordPress installations.
There is a fix for multi-site install details at: http://etivite.com/groups/buddypress/forum/topic/quick-tip-added-banned-email-domain-check-to-multisite-buddypress-registration/#topic
Plugin download: http://wordpress.org/extend/plugins/buddypress-restrict-email-domains/