This script is for the case where your ISP adds a header eg 'X-Spam: Yes' to the message header, 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] X-Spam /^Yes$/i 200 EndBulkScan
This checks for a message header called 'X-Spam' in the incoming messages, and if the value of that header line equals 'Yes', 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.
You need to alter the 'X-Spam' to be the header field name added by your ISP, and the 'Yes' to be whatever text they use to indicate that the message is spam.
Note that the header 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, and use the $ and ^ anchors as appropriate as appropriate