ok so firstly i would implement the Closest GC key just to make sure everythings all ok
secondly i would try using a different System Mailbox, making sure that the system mailbox is on the same exchange server as the journal mailbox, as a lot of times i've seen issues like this where people consolidate the system mailboxes to one exchange server, even though it's meant to be on a completely different server
https://www.linkedin.com/in/alex-allen-turl-07370146