The proper place for the MTA filtering language is the same protocol
that's used to configure the MTA.
The proper place to advertise MTA supported filtering extensions should
also be the same protocol that's used to configure the MTA. I think IMAP
is the wrong place to advertise this because the only place where IMAP
currently has to interface to the final-delivery agent is the mailstore.
If the mailstore is a legacy mailstore, it's quite likely the IMAP server
and final delivery agents will be from different vendors and will know
very little about each other.
ACAP is a reasonable choice for this protocol. If the filtering langage
were broken out into separate ACAP entries for different rules, this would
allow taking advantage of ACAP's inheritance and large list features. I
think that would be quite useful. Ordering can be solved in a number of
ways.