Just gave Zimbra a first look on an virtual FC 4 in VMWARE ESX and it works great.
I am currently running an Squirrelmail Setup for 150 Users (
http://koelnerabend.de ) and I allow user based training of the bayes filter. Due to a lot of stupid users, I had to switch from overall training to a user based setup. With a good training, the user will get up to 97% accuracy.
I seriously consider to change to Zimbra, but I will do some load tests first.
thanks for this great peace of software