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.
Sometimes it is, sometimes it isn't. The point of the exercise is to define
something that can be used anywhere in the process. As such, calling it "MTA
filtering" may not be a good idea. But that doesn't mean the idea is bad.
Ideally filtering will be done at the same time final delivery is done.
Circumstances conspire, however, to sometimes make it necessary to do filtering
at other times. For example, a large ISP may wish to promote user filtering
settings to their outermost MTA so as to avoid the overhead of processing
messages that will later be dropped. Or it may be necessary for a user agent to
implement filtering as part of its internal processing after final delivery
because it cannot count on it being done prior to that point.
in addition, the notion of a "general" filtering language,
as opposed to one specifically designed for email, sounds
like a rathole to me.
I disagree. I think we've made reasonable progress on defining a language and
are likely to be able to reach consensus on it. I also think this, like
firewalls, is one of those issues that the IETF ignores at its peril.
In any case, I would like to try this and see if we can get somewhere.