If you use an email forwarder pointing to your regular email box such as gmail, the spam filtering works as normal. And you know exactly where the source of spam originated from. And it's so easy to cut off the stream of emails.
Today, on my forum I deleted about 8000 inactive user accounts, and switched over to only accepting owner domain email addresses. Since it's a webmaster forum, it should be no problem for webmasters to implement an email forwarder from one of their domains. Oh the joy of not having to check out 1000's of anonymous users.