Fine then, let's go back to basics:
Can you ping the mail server from the outside world?
Can you connect with a pop/smtp client to send/receive email, rather than using the webmail client?
Because it sounds to me as if either your DNS or your firewall, most likely the latter, is eating the mail because as you say there are no bounces.
Perhaps you could use wireshark installed in various places through your network to monitor port 25. I would be looking on both sides of your 5505, and then right up against the Zimbra server, to see the differences in traffic at the three locations. If nothing is making it to the server, you know where to make your changes (or at least have your search narrowed down to something outside the Zimbra server, depending on how complex and extensive your network is).
Edit: You could also try working around port 25, depending on how your ISP is set up. I have one server that accepts smtp from 25 or 2525 due to TELUS' habit of completely blocking port 25 on parts of their network.