I left my last job with around 26,000 emails. I did clean up some of my sent items, and some of the irrelevant email. Perhaps one day I might return to that workplace and role, so emails would be useful.
It was standard practice that for anyone who left, their supervisor or replacement had their inbox added to their account.
As I've worked in IT, I am aware emails can be accessed and restored from backups, I rarely used email for personal reasons, and never ever used it to bitch about coworkers - and would always immediately delete anything personal other coworkers sent me. (and delete from the deleted items)
So many people think that deleting emails will save them - not if they are then sitting in their deleted folder, or they haven't deleted their sent items.
Many years ago a coworker quit on the spot because their manager was a bully. The manager asked me to give them access to the inbox. I refused to do this until I had discussed it with my manager. This was in the early days and IT law has changed a lot since then.
In the meantime, I logged into this person's email account and deleted all their personal emails, and especially the ones between them and other coworkers where they complained about this manager.
I never gave them full access to the inbox - but as the emails had been placed into separate client folders, I exported these and imported them for the manager. They weren't happy about it and I knew they wanted to find dirt on this person.