iredmail-doc/en_US/howto/2-no.x-spam.headers.md

Amavisd + SpamAssassin not working? no mail header (X-Spam-*) inserted

Amavisd config file is different on different Linux/BSD distributions, you can find the correct one for your server in this tutorial: Locations of configuration and log files of major components

Amavisd has below setting in its config file /etc/amavisd/amavisd.conf by default:

$sa_tag_level_deflt  = 2.0;

That means Amavisd will insert X-Spam-Flag and other X-Spam-* headers when email score >= 2.0. If you want Amavisd always insert these headers, please set it to a low score, for example:

$sa_tag_level_deflt  = -999;

Restarting Amavisd service is required after changed setting.