127.0.0.1 localhost.localdomain localhost
x.x.x.x hostname.domain.com hostname
I'm not sure what your setup is - you probably need a Split DNS - Zimbra :: Wiki Another article on BIND: /blog/archives/2007/06/making-zimbra-bind-work-together.html though there's other DNS alternatives it's a common one in the forums.
host `hostname` <---those are backticks
dig domain.com mx
dig domain.com any
(Other option to running a split dns would be to un-check "Enable DNS Lookups" and use a mail relay for outgoing. Or lastly, though not really preferred, modifying the zimbraMailTransport to contain [name] (still requires a good hosts file) or put in an IP address instead, but you'd have to modify it for every user if IP were to change later.)