Preventing registrations from certain email domains
 

A few days ago, I added hotmail.com and yahoo.com to the list of prohibited email addresses under User Banning Options, and checked Yes to allow existing members to keep those email addresses. I got several emails from existing members asking why they were banned, and ended up having to remove hotmail.com and yahoo.com from the list. How can I prevent new registrations with those addresses without bothering the existing members about it?