If a VPOP3 user forwards a message to spam@<your domain>, then the VPOP3 spam filter will train itself that that message was spam.
If, when you forward the message, you get an error message like:
Subject: Mail Delivery Failure Delivery Failure Report. The following message was incorrectly addressed. Recipient: "spam" is unrecognised.
that means that VPOP3 saw the message as an incoming message. VPOP3 does not treat the 'spam@' or 'notspam@' email addresses as a valid email address for incoming messages, only for ones sent by VPOP3 users. This is to prevent other people from 'polluting' the spam filter training by sending messages to spam@yourdomain or notspam@yourdomain.
To be able to send messages to spam@ or notspam@ the sender has to send mail directly to the VPOP3 server using SMTP, either using SMTP authentication, or from an IP address which is specified as “allow unauthenticated access” in the SMTP service IP access restrictions.