check zimbra logs if you found port conflict error log
or
check 25 port by
# netstat -tulpn
or
check 25 port by
# netstat -tulpn
if other program using 25 port then note PID of that program and kill
# kill -9 1285
# kill -9 1285
after killing PID 1285 run
# zmcontrol start
# zmcontrol start