For instance, if the email is BOTH in a geographical blackhole, AND is listed on Spamcop.BL or the like, AND has a high SpamAssassin score, then just kill it, i.e. do not accept it , (and the sender will get a "not delivered" notification, I guess, explaining why).
On the other hand, if the email is OR in a geographical blackhole, OR is listed on Spamcop.BL or the like, OR has a high SpamAssassin level, then let's ignore it, i.e. accept it in the Held Mail.
That is just an example. Even better, each user can set what makes a KILL and what makes an IGNORE, with a set of rules.
For instance
SpamAssassin >20 ---> KILL
Nigeria BlackHoles ---> KILL
SpamAssassin >4 AND Spamcop.BL --> IGNORE
and so on...
What do you think?
Of course, that would require some server resouces