[softawre]

As a manager, if I fire somebody or they leave, I get access to their mailbox via Outlook365 for like 3 months, just in case I need to grab something from it.

So there must be a way to assign ownership of a mailbox to another user.

[Inverte]

Yeah, you can attach mailboxes to other users with Powershell.

You could also just create a mail rule that sends a copy of every email everytime someone sends and receives one and they wouldn't know.