Routing spam to Outlook Junk folder

How are others routing email to users Outlook Junk folders?  We use Exchange on-prem and I am appending the subject lines with [SPAM] and using an Exchange Transport rule to increment SCL based on that tag.  This works but causes an issue when users need to forward email from the Junk Folder, the transport rule picks up the [SPAM] tag and again routes to the next user's Junk folder. The workaround is to edit the subject line to remove the [SPAM] tag before forwarding, not best solution.  Adding "from external users" to the transport rule does not help as Exchange looks at the IntraOrg connector, not the user's email address, and the transport rule does not increment SCL and the [SPAM] tagged email lands in the Inbox.