User A sends message 1 to User B.
When a Worker processes User A's mailbox, Retain archives Message 1 in the Retain archive and creates a record in the database that points to the archived message and associates it with User A's mailbox.
When a Worker processes User B's mailbox, Retain notes that message 1 is already archived and that a database record already exists.
Therefore, Retain only needs to update the database record so that the message is also associated with User B's mailbox.