[colug-432] smtp filters

[Scott Merrill]

I've been kicking around the idea of migrating my email off of Google
Apps. I'd lose a fair bit of integrated functionality on which I've
come to rely, I realize, but I think there may be some long-term
benefits to being responsible for my own communications.

As I think through how I might make the transition, it occurs to me
that most of the anti-spam measures with which I am familiar go about
weighting messages the wrong way. Rather than identify spam, why not
explicitly identify ham, based on my specific corpus of email? I'm not
talking Bayesian statistics, but an actual accounting of mail
exchanged between myself and others.

Specifically, I'm wondering how my MTA might query my mail spool (or
some summary thereof) to see how many messages have been received by a
given sender (both MAIL FROM and envelope from:), and whether or not
I've ever sent mail to that person?  Surely if mail comes from a
specific host, from a specific address, AND I've sent mail to that
same address, that message is worth accepting (content filtering may
still be employed at a later stage).

For a large mail provider, I understand that the SMTP daemon may live
alone, isolated from the final mail store. That's unlikely to be the
case for me, given resource constraints.

It's been a while since I've last deployed Postfix, so the state of
the art may have advanced quite a bit.  Is anyone familiar with a
milter or plugin or other solution that implements anything like the
above? Presumably it would work in conjunction with other more
traditional anti-spam measures.

Any pointers would be greatly appreciated.  Thanks!