Postfix supports a nifty feature named access policy delegation or short policy services. Some of the more well known policy servers include postgrey, SQLgrey and policyd-weight.
These policy servers usually cannot be used on systems running Plesk since qmail is currently the only supported MTA. Since many policy servers provide helpful anti-spam services, I wrote a little script named PolicyHQ (Policy Handler for qmail) which can be used to easily connect to one or more policy servers during mail delivery and process the messages according to their result.
In contrast to some other addons for qmail, no modifications on the qmail binaries is necessary for PolicyHQ. The policy servers can be queried on a per-user (recipient or sender), per-domain (recipient domain or sender domain) or global basis.
Detailed Setup instructions are included in the tarball.
WARNING: PolicyHQ is currently considered beta software and should not be used on mission critical systems. Install it at your own risk.