thanks, it works although now opera opens with a new message-screen instead of the list with email
At the top right of the mail screen, there's a slanted wrench icon on the toolbar. You can click it to change the layout.
Or, if needed, close down Opera and delete autosave.win in the sessions folder. See "Help -> about Opera" for the location.
Latest link is interesting but I can not use because I use osx instead of windows.
If you can find the file on OSX, you should be able to modify it just like you do on Windows.