To monitor your mail server log file, need to access to server by CLI, and run some command as below for details log:
1. Check real time all activities logs
root@mail:~# tail -f /var/log/mail.log
2. Check previous errors, reject, fails log
grep -i "error\|fail\|reject" /var/log/mail.log
3. Check real time errors, reject, fails log
tail -f /var/log/mail.log | grep -i "error\|fail\|reject"
Monitoring email server is not a easy job at all, it take time and effort, in case you want to make your email server great again, just drop us some words, we could consult you more details or service it for you.