If you have any issues in receiving external mails on your Zimbra email account, you have to trace out where the email is failing. For this, send a test message and check the log files on your MTA server.Ensure that you have checked the /var/log/zimbra.log file. Often, it is helpful to tail the log file as given below when you send the message.tail -f /var/log/zimbra.log Once done, check the response. If you see the nothing logged, no connection, or nothing message, the problem is with DNS or your firewall.To resolve the Zimbra cannot receive external mail problem due to the DNS or firewall issue, perform the following step-by-step instructions.
Troubleshoot your firewall issue by having an account on a system outside of your network.