I never tried it until I came across a forum that had that feature where you can set members e-mail to only come from selected domains, for example @gmail.com and @yahoo.com, all other attempts to register with a different e-mail is rejected. Also prevents registered person from changing it in profiles.

Now I understand that would not apply to a lot of forum administrators, but it would be useful especially for those groups such as guilds, clubs, and the such with their own domain name.

That way the administrators will not need to manually add members to a "closed to new registrations forums" and use that feature instead, so members can register themselves in and yet non members are kept out due to no domain e-mail. (it would work with "e-mail verification required to activate account" option turned on)