You have not shown any evidence that your MX record is correct or even exists. Post the output of the following commands (run on the zimbra server):
Code:
cat /etc/hosts
cat /etc/resolv.conf
dig yourdomain.com any
dig yourdomain.com mx
host `hostname` <-- use that exact command with backticks not single quotes
Please note that the domain name in the dig commands should be yourdomain.com and not mail.yourdomain.com (that's the FQDN of your server) and that's probably why the install is failing.