I recently installed Zimbra 4.5.0-RC2 on Ubuntu 6.06. Everything worked except Postfix (mta would not start). I suspect a problem with either my etc/services file (incorrectly allocated ports) or a clash between the ports Amavis and Postfix use. I am not sure which log files I could check, and would appreciate any help. Possibly someone else has had this problem.
In an effort to eliminate the problem, I just upgraded to 4.5.0-GA - without success.