Keep your eyes on the Zimbra project at zimbra.com as they are soon due to release version 4.5 of the groupware application.
It provides administrators with a great deal ofr features within the web front administration section. The previous versions required you to input them all by hand, which is a real nightmare.
I can report that, during the Christmas period, spammers were hitting my clients hard with phishing scams. I am confident that Zimbra is holding up well and will see my through without many problems.
This alone is a distinct advantage over competitors such as Scalix – who seem to resent the idea of including anti-virus into there package. They provide scarce details on how it can be done and say that their application is fully compatible — well why not just do it and earn those valuable extra marks !!!