1. Group support for sender or recipient. We should be able to say "If
the message sender is anyone in group X, then..." instead of
separately listing out all the members of the group.

How else will I be able to block external email to all members of
the Accounts Dept?

2. Filtering on attachment type. Basically, we add attachment MIME type
as one more field in the rule, and if any attachment in the mail
matches the specified MIME type, then we treat the rule as matched.
This will allow us to block certain people sending out MPEG4 movies
but others are allowed, etc.

3. Negation for sender, recipient, attachment type. I should be able to
say "If the sender is NOT this, then ..." or "If the attachment is
NOT of this type, then ...". Without this, our rulesets can become
really long.