Very little in the way of official documentation on upgrading MailScanner on RPM based distributions so here is a 30 second tutorial.

As promised here is an update to our Anti-Spam gateway since we have gone live. We have successfully been running for over a week now and are receiving positive comments regarding the service.
To monitor the performance of the server and to allow for some pretty spiffy looking reports we are making use of MailWatch. MailWatch [...]

While Plesk includes a distribution of SpamAssassin, when you have multiple servers to maintain and a need to integrate everything seamlessly it becomes apparent that this is not the best solution.
Based on the concept of Project Gamera from Atomic Rocket Turtle we have developed an in house Spam and Virus filtering solution that scales well, [...]