jarthurs
The junkmail folder is where messages that have already been identified as spam are moved to. There is no point re-teaching spamassassin about messages in that folder as spamassassin already knows about them (ie those messages that have been automatically moved there by the system, as per settings in the server manager Email panel).
Old howtos refer to creating a folder named LearnAsSpam, & this is the folder that you manually move untagged messages into. The system would scan that folder, train spamassassin & delete the messagrs nightly when done. I am referring to the sonoracom howto, check the FAQ (I think).
So better for you to create & use the LearnAsSpam folder for each user, if not already existing.
There were also other howtos & contribs on the same spam handling subject, so refer to those as well.