Turn on debug mode in Amavisd and SpamAssassin

In Amavisd config file, change $log_level, then restart amavis service.

$log_level = 5;              # verbosity 0..5, -d

If you want to debug SpamAssassin, please update $sa_debug also:

$sa_debug = 1;

Amavisd is configured by iRedMail to log to Postfix log file.