The existing spam handling is excellent, but for one huge missing feature. It would be great to allow a blacklist to actually REJECT emails that match it (by domain, email or IP address), bouncing them back to the sender.
This would greatly reduce the number of spam emails that accumulate in the spam folder, both because some of the biggest offenders would just get bounced back instead, and because after such repeated bounces, most spammers will give up and remove the offending address from their spam lists. Over time, this would save bandwidth, disk space, and wasted time by users.
Note that it is important for such emails to BOUNCE and not just be removed, both because it is an RFC requirement, and because it deals with the possibility of a valid email being rejected.