Apple's Mail Services Update 1.0 [1] for Snow Leopard Server comes with brief release notes, stating only that it addresses "issues affecting overall mail service reliability and performance." Although Apple recommends the update for all users running Mac OS X Server 10.6.2, there have been reports of it causing problems, along with a possible solution [2] outlined in an Apple Discussions thread. It's available via Software Update or the Apple Support Downloads page. Installation instructions [3] for the update are available on Apple's Web site. (Free, 20.59 MB)
[1]: http://support.apple.com/kb/DL982
[2]: http://discussions.apple.com/thread.jspa?threadID=2269047
[3]: http://support.apple.com/kb/HT3975