Thanks, tbirdvet.
As I said, though, I've decided not to delete them. When I attempted to, I got a scary message that I was about to permanently delete "On My Mac". "On My Mac" contains a lot of mailboxes established by me to save old e-mails, as you said, as well as these few that appeared after I upgraded from Catalina to Monterey. When I test the ones established by me, the dialog box identifies the mailbox by name, and doesn't claim I'm about to delete "On My Mac". So, I've moved the ones I was interested in to the bottom of the list where they're less likely to get in my way.
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srb