sendmail's default config does not accept external connections - you've edited /etc/mail/sendmail.mc, right? and run m4 on it?
Assuming you've config'd sendmail:
1) are you filtering using iptables? See if you have rules blocking these ports
2) are you sure sendmail's running? netstat -l (that's an "L")
3) can you send mail from inside the net to external folks? (this isn't a solution, but might identify the real problem)