the width of the mailbox list cannot be fixed

Hey there! Did this happen after updating? Because I don’t think you’re referring to the defunct “Classic View” or Even “Column View” but it sounds more like a glitch, is that right?

I’ve had the same problem, only slightly different symptoms as to where the preferences and saved state of the app never updated.

Anyhow I think this may help:

Always good to keep a backup, however this will not delete any of your mail data, except some logs, note that you may have to set your preferences up again just once and that’s it, (The length of the window, the width of the left column, You May have to select “Show” for some local mailboxes to appear, etc, but it’s usually straight forward), but then it should work!

Hit “Mail” on the top left of the screen, and select “Quit Mail”.

Open Finder. On the top menu select “Go”.

There is a hidden “Library” under the “Go” menu, in between “Home” and “Computer”, however you have to hold down “Option” on your keyboard for it to show.

So anyhow, hit Go, while holding Option, select Library.

In the ~Library, open “Containers”.

If you scroll down, you’ll eventually see “com.apple.mail” (Everything after com.apple is in alphabetical order usually).

Highlight this folder, then on the top menu, select “File” > “New Folder (With Selection)”, then press Return on the keyboard.

Restart the Mac, and re open the Mail app while holding the “Shift” key on the keyboard and test.

If it works normally again after that, quit/relaunch Mail normally and test.

It should respond as your used to then, hope that’s the case, good luck!