Exchange SMTP connections staying active for a long period of time.

I have several clients who are having the same problem.  If you look in Exchange System Manager (both in 2000 and 2003), under smtp current connections, there are a number of domains/ip addresses that seem to stay connected for an extended period of time.  Some for 10-20 minutes, which can slow the server down considerably.   If I disconnect them manually, they seem to just pop back upeither with the same address, or a different one.  Is there a way to have Exchange (or another service) to automatically reject smtp connections that are coming from a known blacklisted source?  Or even if they do not contain a reverse lookup.