If your Mac mail mailboxes disappeared, after upgrading the macOS or moving your data to a new Mac, then you can use the Show/Hide button to resolve it. Apple has added this button to either show or hide a mailbox folder. By default, this button is not visible, which means it is invisible. To access the Show/Hide button, move your system mouse cursor over the folder you wish to show.hide. Now, the Show button will appear and you need to click it.
Now, let’s see the other causes for this issue:
To fix the Mac Mail Mailboxes disappeared issue, carry out any of the solutions given here.