You can decrease value of Amavisd parameter $sa_tag_level_deflt, so that amavisd will insert mail headers in almost all mails which 'X-Spam-Score' larger than it. For example:
$sa_tag_level_deflt = -100; # add spam info headers if at, or above that level
I put that to /etc/amavis/conf.d/50-user and restarted amavisd, but it has no effect. The only thing I found from Google was that the tag is only added to messages whose receivers are local. As this is the case, everything should be ok, but no spam headers appear in new mail that comes from outside to kpelit.fi. I also tried to search for a way to actually see what config values amavis has loaded up (some command for this), but found nothing. Is there a way to do it?
Thanks for support, I appreciate it! 
I really like iRedMail as it has helped me a lot in the past months I've used it and support is very good in this forum.