Amavisd has below setting in its config file by default: File: /etc/amavisd/amavisd.conf
$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 to let Amavisd always insert these headers, you can set it to a low score, for example:
$sa_tag_level_deflt = -999;
Amavisd's main config file is different on different Linux/BSD distributions:
/etc/amavisd/amavisd.conf
/etc/amavis/conf.d/50-user
(and other config files under /etc/amavs/conf.d/
)/usr/local/etc/amavisd/amavisd.conf