After installing Big Sur I had a first look on how Mail Archiver works. Almost everything is fine.

There are only a couple of visual glitches.

  1. In the main window the delete icon in the toolbar is smaller than the other icons. There is a small gap between the toolbar and the rest of the window.Main window in Big Sur

  2. The Pro Admin version has a couple of black in black listbox headers:Listbox headers in Big Sur

  3. What looks very strange is the toolbar in the Preferences. The selected item it hard to see. I opened a bug report with Apple. But they only employ children with perfect eyesight. If necessary I can change the icon of the selected toolbar item.Toolbar of preferences in Big Sur