We also share the same O365 Tenant but with our own AD domain ... (theirs), and trying to figure out how to "move" the mailbox to the new AD account. Any idea if this is supported or possible?

Right now our retention policy is infinite with all emails going to a journaled mailbox, now standing at 26GB. I am looking at a couple solutions like Symantec Vault, Symantec Email Archive (SaaS ...