Apple has rereleased the OS X 10.6.5 update for the Server version of OS X. The update was initially available for download along with the client version, but was pulled a short while later with no explanation by Apple. This left people speculating about the reason on discussion forums.
Apple has now rereleased the update, and along with it issued a knowledgebase article stating the new release fixes a problem with the Mail service, where memory mismanagement would lead to mail being sent to or at least be read by the wrong account.
The rereleased update is available as a Combo update (1.12GB) and a Delta update (858.02MB). If you have not yet updated OS X Server, the update should also be in available in Software Update either when you log into an account or if you log in using Server Admin.
If you already installed the initial OS X Server update, you can reapply the new version, which will patch the Mail service bug. We recommend you use the Combo updater when doing this.