Personally, I think any filtering should be done by
the MTA, and I see little applicability in keying some filtering off of the
opening of a mailbox.
Ideally yes, but if the MTA does not implement/permit filtering or
does not advertise the necessary hooks for the client to install filters,
the client has to do the filtering on its own. And this is typically
triggered off during the opening of the INBOX.
It would be nice if the filtering language does not assume that it
is always executed only by the MTA. Anyways I think MTA side filtering
capabalities are typically a subset of client-side filtering ....so shouldn't
be a problem ..
-john