netstat -tanpl|grep 25
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
If you were root, then how does this happens?
Are you sure that sendmail is working on runlevel 5 and not on runlevel 3? Did you try to restart it once on rc3?
What are the logs (/var/log/messages, /var/log/maillog etc) saying?