The mailboxes (pst files) are separate from the mail accounts. Sure you can have IMAP, etc, but what's on the PC is local content, same as if you downloaded files from OneDrive and saved to a local folder. That's where the security is different. Unless the PST itself were password protected, it's just a file siting in a Windows directory.
For instance, you could create multiple mailboxes yourself on your local PC, regardless of what email account you use, and drag/drop items into it out of other boxes tied to accounts. then later, you could still open those PST files without your login credentials.
I guess it's assumed that if you're downloading the PSTs to your computer, then why wouldn't you later have the same access? You'd have to login with Windows to get access to the files anyway, so the same or similar security would be at the OS level. and if the PST files are on your PC, then you had permission in the past, one way or another.
Another example: you are migrating from one email service to another. You want to keep the old mailbox, but have no right to keep old account login credentials. The mail stored in your PST is your mail, whether you continue to pay for the subscription service further or not. So you keep the PST file (mailbox file) and delete the account from Outlook.
Basically you have to remember: the mailbox file and the account are 2 separate entities. If you want or need more control, then you have to think about other ways of doing so.
"But thanks be to God, which giveth us the victory through our Lord Jesus Christ." 1 Corinthians 15:57
