On Tue, 16 Jun 2015 17:31:07 +0000, you wrote:
By default, most MTAs only listen to localhost, so don't accept externally derived mail. Try telnetting to port 25 on the machine from some place off your machine's network. If you get a "hang", it's likely a firewall issue. If you get a "connection refused" message it's probably that the MTA isn't listening on the external interface.
If the MTA responds, do a manual delivery exchange and see what you get.
If you're not seeing anything in your maillog, this is very likely an issue with your MTA's configuration, not dovecot.
Now that I think about and concentrate on what you wrote, make perfect sense. Add to that the fact that I received bounces from my ISP that all said the same thing: Unable to establish an SMTP connection. The return code was 4.5.1.