The original version of the Mac OS X 10.6.5 Server update, released by Apple on 11/10/10, apparently has several bugs. Many early adopters are experiencing mail being delivered to the wrong address, DNS resolution issues, LDAP/Open Directory master problems, and local/remote volumes drops.
Apple pulled the Mac OS X 10.6.5 Server update on 11/15/10. The next day, they re-released the Mac OS X 10.6.5 Server update, now marked v1.1. Apple claims they fixed a memory aliasing issue in Devecot’s handling of user names. This is supposed to address the mail being delivered to the wrong address problem, but it hasn’t been confirmed if the other 10.6.5 problems have been fixed.
I would recommend staying away from the Mac OS X 10.6.5 Server update, and wait until the 10.6.6 Server update is released (already being seeded to developers). If you have already updated to Mac OS X 10.6.5 Server prior to 11/16/10, and are experiencing any issues, it would be a good idea to download the Mac OS X 10.6.5 Server v1.1 combo update and re-apply it.