(A) Your mail server is probably configured as 'mail'. This works when connected directly to your cable modem but does not work when behind sharing software or hardware.
Try a fully qualified domain name instead of the IP address or just 'mail'. @ gave you a domain name for your computers. It should look something like ncity.nstate..com
For your mail server, you would use mail.ncity.nstate..com
Of course don't use ncity and nstate.. You will be able to find your own domain name in some of your e-mail headers.
You can also try hooking your computer back up to your cable modem directly and type 'PING MAIL' from a command prompt to get your real mail server address.