Somebody has left and their Office 365 licence is about to be pulled
Removing the licence starts a clock. Microsoft 365 holds the mailbox data for thirty days after the licence goes and then deletes it, whether the mailbox sits in Exchange Online or on premise, along with anything HR or legal might ask for six months later. The mailbox has to come down to disk before the licence goes, not after.
Worth knowing. Do it in this order: export, open the export and check it, then hand the licence back. Reversing the last two is how mailboxes get lost, and it is not recoverable once the seat is reassigned. How to archive old Mac email to an external drive covers checking the copy before anything is deleted.