This should work for e-mail forwarders and also for mail sent out through the server. I did also run some tests and this does appear to be the case (although very limited tests). This is because any e-mail that comes into the server (whether it is being sent out or being received from another SMTP server) would be checked for spam. After the DATA command and after the message is completed, the message is scanned for spam. If the SpamAssassin score is above the limit, then the message is rejected and not accepted. This means that if a user sends a message with their e-mail client through the server and it is detected as spam, the user should receive an error stating that the message did not get sent. Likewise, in the case of an e-mail forwarder, the message is rejected before it is accepted on the server and forwarded on to the forwarded address.
I can see where this would have real promise in helping with e-mail forwarder spam as well as sending spam and just generally any spam. I haven't yet implemented this, I am still running some tests and I am keeping an eye on this thread for user experience.