This is in HTML at http://canonical.org/~kragen/sw/minimilter.html.
This is a very simple milter implementation in pure Python, including a sample milter. I'm using this to filter spam under Postfix on panacea.canonical.org, which is Bad and Wrong for several reasons:
Milters are a way to reject or modify mail before it goes into the mail queue. I care about this because it lets me bounce spam to its real sender, not the forged from-address on the mail, so I don't become part of the backscatter problem.
MTAs that support milters include Sendmail, Postfix, and qpsmtpd.
libmilter, which lets you write milters in C.
I'm using the sample milter to bounce unauthorized mail to some Mailman mailing lists, on a machine with Postfix, so that our server doesn't generate backscatter. I'm probably going to switch away from it shortly because it turns out Postfix has a built-in feature that does more or less the same thing.
Backscatter is when spam sent from a forged
From address gets
bounced back to that address, effectively turning the spam's first
recipient into an inadvertent spam source. To avoid creating
backscatter, don't bounce mail after accepting it; instead, refuse the
mail at SMTP-time.
git clone http://canonical.org/~kragen/mailman-milter.git
Kragen Javier Sitaker email@example.com