Compatible XF Versions:
Allow only specific e-mail hosts to be used in e-mails or disallow others.
This add-on works not only on user registration but on other pages such as the user's Contact Details page, when a user modifies their e-mail.
Administrators can still bypass the e-mail restrictions when editing users via the Admin Control Panel.
This add-on is sponsored by Case.
Please let me know if you encounter any issues or have found it useful!