SMTP 25 closed even though I allow everything in firewall (Debian server running sendmail)

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

DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl

in your 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.