This repo has three python milter packages.
This is the original application that drove the need for pymilter, and then pydspam, pygossip, pyspf, pydkim. It is very effective,and used on dozens of production mail servers. But it is kind of "hacky", with features added by busy mail admins without enough thought toward ease of configuration by new users. Even if you don't install it, I recommend it as a rich source of examples of real life use of pymilter and other email related python packages.
The driving principle is authentication and reputation. For instance, when banning a domain, you don't just ban the domain, but a given level of authentication for that domain. If spam.com is unauthenticated, then banning spam.com does not block an authenticated email from spam.com - because the spam may have been forged. But if the spam email from spam.com that triggers the ban (or bad reputation) is authenticated, then all emails from that domain are banned.
The primary authentication methods are SPF (Sender Policy Framework) and DKIM. DMARC would be a future enhancement. Greylisting is also used.
This is a simple SPF milter that adds Received-SPF, logs, and optionally rejects based on reject with policy configured per MFROM domain. Use as is, or as a starting point for your own custom milter.
This is a simple DKIM milter that signs outgoing mail and adds Authentication-Results, logs, and optionally rejects incoming mail based on local and Author Domain Sender Policy (ADSP).