Why is being sendmail daemon killed on Red Hat Enterprise Linux 5?

Issue

• Sendmail daemon is being killed after restarting it. First it shows following :

[root@]# service sendmail status
sendmail (pid  8455) is running...
sm-client (pid  8465) is running...

• After somtime it shows following :

[root@]# service sendmail status
sendmail dead but pid file exists
sm-client (pid  18282) is running...