There is a known issue with Outlook and Exchange where "secondary" mailboxes which are mounted in the same profile, e.g. "John Doe" + CorpMailbox1, CorpMailbox2, CorpMailbox3 will place "sent items" from the secondary mailboxes (e.g. CorpMailbox1) in the user's (e.g. "John Doe") Sent Items folder.

There is a registry value (something like DelegateSentItems) to semi-fix this, but I'm looking for an Exchange-side solution.

It seems like, at some point, an admin made CC rules to fix this, and for the mailboxes where I see a BCC rule: any item sent to [address] BCC to [address].

The behavior "fixes" the issue, but when I duplicate this work-around for new shared mailboxes, the email ends up in the Inbox and not the Sent Items for the shared mailbox.

I don't believe there is any outlook/client-side shenanigans going on, as the intended behavior for the older mailboxes works for new PCs/Users (new imagse) without any Outlook Mailbox Rules.