Here is a little tutorial for how you do it. Its actually pretty simple:
Plesk's qmail does not set the mail header "ESMTOA" correctly as per RFC3848 (http://www.fehcom.de/rfc/rfc3848.txt
). To get around this you can create your own spamassassin custom rule to detect your own mail server tag match to LOCAL_AUTH_RCVD and apply a negative score if need be.
Edit your local spamassassin config (/etc/mail/spamassassin/local.cf) file and add the rule and new negative score there (sa-updates will never overwrite it in local.cf).
# custom rules
header LOCAL_AUTH_RCVD Received =~ /by yourhostname\.yourdomain\.com with \(.* encrypted\) SMTP/
score LOCAL_AUTH_RCVD -10.0
Substitute your hostname and server name with your own.
The first line is just a Perl regex to match your own server hostname . The wildcard match will match different authentication schemes (CRAM MD5 etc). You can look at your own mail headers when you send authenticated to see what they look like. A good regex tester is here : http://www.spaweditor.com/scripts/regex/index.php
I hope this helps people!