... all of which confusion, and the fact that my prehistoric backup system missed the (one and only!) .PST file is why I abandoned Outlook to its fate.
I did find the lurking .PST but it was too late.
Thunderbird puts the mail into a directory that I choose (I:\Mail, me being a bear of little imagination). Within that are subdirectories for each e-mail address, plus a few other items such as address book. Each mailbox subdirectory has files for inbox, outbox, and the various filed messages categories.
It's easy to drive, backs up up quickly, is unlikely to be damaged - and if it is, the backup for that specific file drops straight in. None of the other mail is affected. Each to his own, but it works for me.