No, not jailbroken. I don't recall ever using Mail on my iPad yet, but I might have once or twice, I don't recall. So I haven't made any folders other than the default.
OK, on about the sixth try, Mail opened to show all my Mailboxes. It was just showing a generic blank Inbox before. So I was able to select the 50 old messages and delete them. Since I had long ago already read these messages on my Mac, this is obviously redundant. Am I always going to get all my mail on both devices, regardless if they have been read already on another device?