When trying to find the email logs on my Plesk server I struggled to find them at first.
Parallels Plesk Panel maillog is in a non standard location compared to you standard Linux system.
On most Linux systems you will find the mail log in /var/log. However Plesk Panel stores the logs in usr/local/psa/var/log. If you want to see the servers mail log use the following command.
tail -f /usr/local/psa/var/log/maillog
I recommend creating a symbolic link to plesk logs within /var/log to make it easy to locate in future.
ln -s /usr/local/psa/var/log /var/log/plesk-maillog