Firstly, with thanks to symcbean, check it's running. On the system itself, type
telnet localhost 25
and if you don't see something like
220 risby.home.domainname.net ESMTP Sendmail 8.14.4/8.14.2; Wed, 1 Dec 2010 10:20:38 GMT
your sendmail isn't running in daemon mode, and you won't be able to get to it from anywhere. Fix that.
If you do see a banner, then it's likely sendmail's not binding to your external IP address. Do a
netstat -an|grep -w 25|grep LISTEN
If you see the line
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN
I'm wrong, and we must dig further. If you see
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN
then sendmail has been configured only to listen on the loopback interface, which is why you can't get to it from off the box. Look for a line that says
sendmail.mc, delete it (if you ignore me, and comment it out instead, use a
dnl at the beginning of the line, not a
#), regenerate your
sendmail.cf and restart sendmail.