No need to apologize. My annoyance is directed at Apple for using the word "rebuild" when they mean "delete". I think that's just atrocious. If they want to offer an option to actually "rebuild" the index, then they should provide that.
Just make sure your e-mails are secure on the server.
If Mail is still confused, then delete your account from Mail. Then, re-add it. Then, wait. Wait some more. Keep waiting. I don't know how much e-mail you have, what kind of internet connection you have, or what kind of computer you have. But this can easily take days to complete. There is no progress display of any kind. You simply have to wait.
I recommend a little app called "Jiggler". It will "jiggle" your mouse and keep your computer from sleeping. If your computer goes to sleep, all of this stops until it wakes up again. So you have to keep your computer plugged in, "jiggled", and downloading e-mails until it completes.
The "index" in Mail is simply part of the larger, whole-disk "Spotlight" indexing system. When it works, it works great. When it doesn't work, it is absolutely infuriating. It will need to reindex the entire computer on a regular basis, without any warning or explanation. If all of this is happening while you are trying to install a new operating system like Ventura, that only adds to the time you need to wait. After a major operation like Ventura, the system needs to sit for a while (ideally kept awake and "jiggling") while it rebuilds the entire Spotlight index. And then if you have also tried to "rebuild" your Mail, you now have to download potentially several gigabytes new e-mail data and reindex all of that. Wait. Wait. Wait and then wait some more.
