The proper place for anti-spam integration is the MTA, not Mailman. If you want to improve what Mailman can do as a response to what the anti-spam software has done to the incoming messages, that's great – but that's not the same thing as integrating or providing hooks for an anti-spam solution.

That said, it may also be useful for Mailman to provide hooks to integrate various anti-spam solutions. The standard Pythonic anti-spam solution would be SpamBayes, but I think we should definitely do what we can to support others as well.