iRedMail provides DKIM singing and verification with Amavisd-new. To enable or disable DKIM, you should edit amavisd config file.
On RHEL/CentOS, config file is /etc/amavisd.conf, on Debian/Ubuntu, it's /etc/amavis/conf.d/50-user, On FreeBSD, it's /usr/local/etc/amavisd.conf.
To enable DKIM signing and verification, edit config file and make sure you have below two lines:
$enable_dkim_verification = 1; # enable DKIM signatures verification $enable_dkim_signing = 1; # Enable DKIM signing
To disable them, set value to 0.