I would say that in this case... the personal stuff should have stayed off of the work email.
I'd rather leave a company knowing that my accounts were locked, than not. If it's locked, I have more confidence that I wouldn't be accused of doing something seedy when packing up.
Shoulda woulda coulda. Yes, people shouldn't send things to work, but they do. All the time. Especially so if the event happens after work, for example. Or maybe it's part of an overarching todo list you store in Exchange. Or maybe you have a few documents in OneNote which are personal but you hadn't had time to send them to yourself. It happens quite frequently. The implication is also that the account will get locked, for example, at the end of the day, so they can wait until the end of the day and then see their account locked and leave.
I'd rather leave a company knowing that my accounts were locked, than not. If it's locked, I have more confidence that I wouldn't be accused of doing something seedy when packing up.