Milter-greylist

> I wouldn't tie that to the hostname but to the MX records:
> If you got an e-mail from "user@..." from IP address 1.2.3.4
> and at least one of the MX records for "domain.com" points to a host
> whose name resolves to 1.2.3.4 it is probably not spam.

This would probably be even more efficient, but the MX record
is for receiving mail, and it doesn't have to match the machine
which actually sends mail for this domain (although it often does). 
milter-greylist would have to make an MX lookup, while it already 
gets the hostname and sender from sendmail. This is more a job for
*real* solutions like SPF/MTAmark/etc.

I'd like to emphasize that my proposal is not a solution to
actually stop more spam at the greylist barrier. It's for those
greylist users who have a problem that lots of legitimate messages
are deferred or stopped by greylisting. It should make greylisting
be more acceptable even without local whitelists, while not letting
(much) more spam through. And spam that matches the new rule is
probably coming from only a handful of domains which can then be
blacklisted easily.

mp.