[WhaLberg]

Hanoifishing, yes, it is possible with a plugin. Add these codes to register_form_complete hook.

PHP Code:

// allowed email address.
$domain = "domain.com";
// check if the user is using the allowed email address.
if (strpos($vbulletin->GPC['email'], $domain) === false)
{
    eval(standard_error(fetch_error('you_are_not_allowed_to_register')));
} 


Add a phrase under "Error Messages" and enter "you_are_not_allowed_to_register" for varname and enter your message that you want to appear or feel free to change the error message.

That's all. Good luck.