You may want to consider disabling the "ConfigServer Mail Queue" plugin and using the cPanel mail queue option instead:
"WHM >> Email >> Mail Queue Manager"
We've made significant improvements to the functionality of this option in version 11.32. As for the lost emails, you can check the following log file to determine if they were delivered:
You may need to search for specific entries, so using "exigrep" may be useful. EX:
Simply replace the "email@domain" entry with an actual email account or message subject.
# /usr/sbin/exigrep firstname.lastname@example.org /var/log/exim_mainlog