Mail getting stuck in Queues
There are circumstances where mail gets into the server but it does not get delivered to the recipients.
This is likely to happen if you put Zimbra behind a firewall and port forward to an internal address. This is as your DNS mx record will resolve to the outside address.
This is fixed by using a DNS server on the private address side of the firewall and setting the A and MX records for the domain to point to the local internal address. It's important to understand that the Email System (SMTP) that sends and receives messages is it's own program, bundled with Zimbra. There's a point where the Email System passes the mail off to Zimbra, and it relies heavily on DNS to do this. Please see: 
Another case is when the amavis tmp directory becomes full (it is currenlty only 150Mb)
This is incomplete as there are other cases in the forums, This section needs update