Stop and start postfix. Do not restart, use individual commands for stop & start.
If you are still facing the problem
Remove /var/spool/postfix/pid/master.pid and /var/lock/subsys/postfix
Stop and start postfix
If you are still facing the problem, Check what is running on port 25
If you are still facing the problem
Remove /var/spool/postfix/pid/master.pid and /var/lock/subsys/postfix
Stop and start postfix
If you are still facing the problem, Check what is running on port 25
# netstat -lnp | grep :25
Kill the process and start postfix again.
