Just my $0.02. Have you considered adding greylisting. I find the combination of greylisting and Spamassassin with the SA's bayes filter completely handles my spam problem. On my primary MX I use spamd on OpenBSD and on my secondary MX I use spamd on FreeBSD. As a very informal method of measurement my Inbox.spam folder, held an average of 400 messages per day in October before I started using spamd. It currently averages about 80 messages per day.

If you don't know about greylisting it works as follows. A greylister monitors port 25 for inbound mail connections. When a server connects to this port to exchange mail the greylister predetermines the response based on whether or not this server has exchanged mail in the recent past. If it has it's allowed to exchange mail again and the server's timestamp is updated. If the server has not exchanged mail in the recent past the greylister responds: "45x - I'm too busy to talk to you right now. Please try to deliver this mail later". It then puts the server and information about the mail being delivered onto a list. If the same server tries the same message later it passes and the greylister promotes the server onto it's list of okay mail servers (mail servers that it has exchanged mail with in the recent past).

Greylisting works because many, and I'd like to say most, spam programs never retry message delivery. The best thing about greylisting is that combines well with filters like SA by reducing the amount of mail that they have to see. In my case something like 80% of the mail that Spamassassin used to process just never gets past the greylister today.

The downsides to greylisting is that it delays the first message from a legitimate mailserver. In the most common case the incurred delay will be between 30 minutes and an hour. This assumes that then sending mail server retries queued mails every half hour or so. In an extreme case the delay may be longer. If the mail sender has a cluster for delivering outbound mails and that cluster features shared message storage and several processing units to handle the smtp transfer then the greylister will trap that message until the same server attempts redelivery. This is a problem with mail coming from very large internet companies like Google or AOL or very distributed corporations like General Electric, Unilever or United Technologies.

Since you are in an ISP environment greylisting may not be something that you can do. I was extremely surprised when a client told me that the 1 hr delay in receiving mail from new and infrequent mail servers was too much to pay to stop the spam coming into his mailbox. I don't claim to know the political layer as much as I do the technical one.


-- Chris

--
      __o          "All I was doing was trying to get home from work."
    _`\<,_           -Rosa Parks
___(*)/_(*)___________________________________________________________
Christopher Sean Hilton                    <chris | at | vindaloo.com>
        pgp key: D0957A2D/f5 30 0a e1 55 76 9b 1f 47 0b 07 e9 75 0e 14
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