I am running Zimbra 8.0.3 on Ubuntu 12. This is a test environment. I have successfully installed and configured Zimbra with only one exception, outbound mail via an external MTA. What is strange is that certain emails are passed on from the external MTA to the correct destination, others are never delivered but no errors/messages bounce back nor am I turning up any errors in the Zimbra.log. We use our domain host to handle company email (Bluehost). Any emails sent between company email accounts via the domain host mail service are delivered to the proper destination; email@example.com to firstname.lastname@example.org. Any emails outside the company are not and they do not bounce back; email@example.com to firstname.lastname@example.org. Email retrieval for each employee is still through the domain host and delivered to the Zimbra desktop client each employee has running on a workstation that retrieves mail via IMAP.