Milter-greylist

Right now, milter-greylist optionally uses SPF for one thing only, whitelisting
hosts that pass SPF checks.

This makes sense in the standard "greylist everything with some exceptions"
configuration of milter-greylist.

However, that's not how I use milter-greylist. I default to whitelisting, and
only greylist specific hosts.

I'm envisioning something a little more flexible.. something like making SPF an
ACL target so you can do things like:

acl whitelist spf pass

or, more for what I'd be interested in:

acl greylist spf fail

Does anyone think that would be desirable/useful? I might be able to put some
time in and develop a patch to do this if there's interest..