I've installed Zimbra Collabration Suite 5.0 on Ubuntu Server.
When I boot the server and check
zmcontrol status
I will notice that everything is running except for MTA

MTA stopped
postfix not running

When I check the mail.err in /var/log/mail.err.
I've noticed a message : postfix port 25: Address already in use

It seems that when you've installed the program "mail" (apt-get install mail)
It installs by default a little mailserver "exim4", exim4 is a mailserver that is used for internal mail but uses port 25.

apt-get remove exim4

And you will be able to start postfix/mta