Hi all. I'm trying to get my head around how to setup MX based spam filtering. Specifically, sendmail + procmail + spamassassin (I'm assuming) is the best way to accomplish this. I want to point the MX record of a couple of domains at a given mail server, have it mark spam as such by modifying the subject line or possibly quarantining spam into a local mailbox as a catch-all for the entire domain's spam, then passing on the clean mail to the "real" MX for the domain, which would be unknown to the outside world.
I've read that /etc/mail/access is one way to accomplish this, but it seems to me that if I define 'mydomain.com RELAY' here, i'll likely be opening up the filtering box to relay spammers who forge their From address as firstname.lastname@example.org
(assuming spamassassin does not catch it), not to mention that sendmail still does not know where the real MX is. There is also FEATURE(`relay_based_on_MX') which would presumably allow anyone on the Internet to point their MX record at my filter box and get their mail filtered at no charge (which is also not desirable).
Can anyone shed light on how to accomplish this? If there is a guide somewhere that describes how to set this up, whether for Fedora or otherwise, please advise. Thanks in advance!