Try going to http://192.168.100.1.

A majority of cable modems run a server there. Try this both directly attached to the cable modem and behind the router.

It's one more datapoint, in a pretty confusing issue. My only guess is the router is not passing through web traffic for some odd reason.

Also, try this telnet command:

telnet empegbbs.com 143

You should see back * OK Dovecot ready. Thats the mail server answering on this server, and will help narrow down exactly what the router is potentially blocking.