Matt,
I'm very dubious about the desirability of standardizing an
MTA-level mail filtering language.
with few exceptions, the mail transport system should not be
doing mail filtering. the mail transport's job is to deliver
mail, not to make filtering decisions on behalf of the user.
if recipients want to filter their own mail, that's their
business, but then it's a UA-level thing.
in addition, the notion of a "general" filtering language,
as opposed to one specifically designed for email, sounds
like a rathole to me.
Keith