Turn on debug mode in OpenDMARC

In OpenDMARC config file /etc/opendmarc.conf, find parameter MilterDebug like below:

MilterDebug 0

Set its value to 1 and restart opendmarc service to enable debug mode:

MilterDebug 1

It's configured by iRedMail to log to /var/log/opendmarc/opendmarc.log.