This script is for the case where your ISP adds a marker eg '[spam]' to the message subject, and you want the VPOP3 spamfilter to quarantine messages with that marker.
Add the following to the spamrules_userchecks.txt file. This script file only gets processed on incoming messages (the spamrules_localmail.txt script ends the processing of locally sent messages)
DefineRule "ISPSpamMarker" 1 "Rule to add score to if the ISP marks the message as spam" BulkScan Score [ISPSpamMarker] Subject /^\[Spam\]/ 200 EndBulkScan
When the subject starts with [Spam], then the spam filter will add '200' onto the spam score (the default threshold for messages to be marked as spam is 100) - you can alter the 200 at the end of the line as appropriate for your requirements.
Note that the Subject check uses regular expressions, so if you change it, make sure that you 'escape' any special characters (eg [, ], *, ., etc) by preceding them with a \ character