Deleting Preference and Other Files

I'm not sure what else to do. What would you suggest next?

Use it to determine if it works better.

A clean install is normally not necessary. The vast majority of users simply upgrade with everything in place. Clean install is sometimes done when there are indications that the previous system has issues. What OS are you currently on? Some users always do erase/format/clean installs for any OS upgrade. I have only done that once, the reason being that the old system was hanging on a Security Update, which is an indication that the existing system has "issues."

I would like to re-emphasize that having more than one reliable backup is important before a major upgrade to the OS.

I had suggested being cautious about deleting files ... I suggest again not deleting files.

If you did not empty the trash, move them back to their exact previous location from the trash.

If you emptied the trash, then you can restore them from a backup, if you know exactly what you deleted and exactly where they were.

If you don't know exactly what you did or where those files were, then you can just live with it, or if you want to correct it, you can try to restore the entire Mail folder that you deleted the files from. Typically the Mail folder is inside your user Library folder. I don't know what exactly you deleted, where was it and what was it?

QUESTION: Doesn't reinstalling the OS through the Recovery HD put a brand new version of the OS on the HD?

Yes, but configuration files, preferences, etc. are your data and are not deleted by the reinstall.

Would installing Catalina over this not solve the problem?

Most likely not for the same reason noted above.

Is a Clean Install called for?

Might be worth a try to completely erase the disk, reinstall the OS, create a new user name that you have never used, and set up your email accounts from scratch.

See what happens. If all good, you can try to Migrate your old user account over and see what happens with that account.

I guess for that matter, you could just create a new user and add your email accounts to it. See if you still have the same issues with just a new account.

I don't know what you mean by Deleting the Envelope Index will cause Mail to "Rebuild" the mailboxes by digging through all of the raw messages stored on your Mac.

You asked about deleting configuration files. The Envelope Index is the database that Mail uses to display and rapidly search for email messages. It doesn't scan through all of the raw text and display that. Mail gathers all of the information from the raw messages and stores it in the Envelope Index database file. It uses that to build the lists of emails. When you select one, it looks up the pointer to the raw message file and grabs it for display.

If that gets wonky, you'll end up with blank messages, incorrect headers for the message, or other weirdness.

I'm not sure that is necessary at this point.

Mailbox > Rebuild causes it to re-index the mailbox you have selected. Deleting the Envelope Index is effectively the same as rebuilding every mailbox.

Deleting the Envelop Index (including -wal and -shm) will force Mail to rebuild the index when you restart Mail (delete it while Mail is not running).

I do have stored messages in "On My Mac" mailboxes. Could I remove them and then reinsert them if Mail was rebuilt?

If you export them to a folder in Finder and then re-import them. I would export each folder individually as it makes it easier to import later on. When you import, it puts the messages in an Imported Messages folder. You then have to move them to your desired folder (which you have re-created).

I think there is an undocumented limit of about 200 messages for each export. It doesn't tell you, it just doesn't export them.

I don't understand how to do this: you can just "reset" your email accounts by signing out of the email accounts in Internet Accounts system preferences (AppleID for iCloud), then add the accounts anew.

For an iCloud account, just select the account, then deselect Mail from the list of iCloud services.

Your ON MY MAC folders will remain, even if they were messages from your iCloud account. You would only need to save them if you erased the ~/Library/Mail folder.

In your screen shot, you do not have your iCloud account selected. You have an AOL Instant Messenger account selected.

If you want to access your POP accounts sometime in the future, don't delete them and don't erase your ~/Library/Mail folder.

~/Library/Containers/com.apple.mail/

Would the Mac OS rebuild this? What would be lost, if anything?

You would lose all of Mail's settings including Favorites, what folders are displayed, etc.

I would only do this as a last resort when nothing else works.