I'm using iRedMail and im wondering if I can use only DKIM verification and dont signing emails with DKIM??
You can disable DKIM signing in amavisd config file, then restart amavisd service:
$enable_dkim_signing = 0;
Amavisd config file could be one of below:
Also, iRedMail has its own support forum: http://www.iredmail.org/forum/ You can always get quick response in the forum.