Users reported that mail stopped being sent from their server.

Tried to test email functionality from the command line.

# echo "Subject: Testing Sendmail from server.domain.com" | sendmail -v admin@domain.com
This is a test
BYE
[ctrl + D]

The message wasn’t sent. I tailed the maillog &  checked the service status:

# tail -n 500 /var/log/maillog
# service sendmail status
master dead by pid file exists

I checked to see if any Sendmail processes were running and there were several.

 ps -ef | grep sendmail

Stopped the service, deleted the master.pid and the lock file and killed all the sendmail processes, then restarted the sendmail service

# service sendmail stop
# rm /var/spool/postfix/pid/master.pid
# rm /var/lock/subsys/sendmail
# killall -9 sendmail
# service sendmail start
# service sendmail status

Tailed the logs, resent the email and it was delivered.

# watch tail -n 20 /var/log/maillog