user A send email to user B
User B answer, User A anwser.
At some point include User C in the thread, and keep sending answer between them.
user A + User B can see the thread as conversation.
User C can NOT see the thread as conversation. (thread is incomplete).
Request for enhancement:
Clear the $REF field for User C enter at mid conversation on first reception of email.
Either modify template of mailbox mechanism.