Yes, as I do not like the poor abilities of Mac OS X Server Admin app for server configurations, specially as a mail server, I have a virtual freebsd box running under Parallels in a production Xserve.
Apple has "reconfigured" postfix, amavis, etc. and keeping them updated, specially the amavis-new, depends on Apple updates which are always obsolete when issued.
So I tried to run a freebsd running the latest versions of everything under Parallels and, after a couple of hours I had a mail server running latest stable versions of Postfix, Dovecot, Spamassasin and Amavis-new.
So now this Xserve is running as a Webserver under Mac Os X and as mail server under Parallels plus freebsd 6.2. Freebsd is using its own IP address.
Working with about one hundred mail accounts and no problems at all since I started it a copuple of months ago.
I know this is not the best solution as Parallels is a desktop (not a server) based virtualizer but it works and the results are better, by far, than using postfix in Mac Os X.