Block single virtual domain from sending email to a specific domain

Hi there

I have a forum that is very busy and generates a lot of attention from people trying to register...generating a lot of email to yahoo email addresses. As the server now has a bad rep with Yahoo..I want to block any mail being sent to Yahoo from this domain. I need this to be for proper mail users but, more importantly, I need it to be blocked from PHP sendmail.

Is there a way I can do this? If I can't do it for a specific virtual domain, then how could I do it server wide?




Thanks - I have already configured the forum not to allow registrations from Yahoo, but I want to restrict those that have already registered as well.

I will check out the link