I have been using Clarkconnect firewall server (clarkconnect.org) and it has features like Spamassassin. You can import your mails from other mail server and then do spam check to your mails.
From there I got this idea marking spam for ISP config.
Ill explain it here and add picture from clarkconnect's spam interface.
Take a look at picture included.
There is a "Quarantine - Score" where you can choose scores.
what is low, medium, high or very high spam.
Then you choose actions for that.
if its low, low - medium, medium - high, - high - very high or very high - choose an action Discard, save to directory, pass throught or send to admin.
This is very smart for handling spam! when now you can only choose pass or discard. And if you pass and rewrite subject you have to do rules for your mail program for putting everything marked as SPAM to go to a different folder.
I have been using Clarkconnect as my client's firewall and antispam machine and I havent got false positives, when spamassassin says score is over 10 you can surely delete is from server, cause its SPAM for sure. This system will decrease you SPAM mail flow giving it a better management.
- Maxx (I have ideas, no skills of coding