Hi Grimnir
Thanks for posting and welcome to the community
Graham is right here it does sound like an old account that was not moved over or deleted when you originally left, as mailboxes normally are after 90 days.
We can try to help do this via what is a Move and Transfer process. This requests a move from the old account to the new one but would delete any new emails you've set up on the new account.
We would also need to be dealing with the account holder for this as well, as something that only the account holder can do. I'll pop you a PM over now to get some details
Best,