Back when graphical interfaces were invented the individual elements had meaning. You could see that buttons were meant to be clicked because the shape was familiar. Shadow was meant to show which window was in front of another window.
Since a few years a lot of meaning has been lost. We now have sort-of transparent windows. What benefit has the transparent window except making odd flickers?
Differentiating between clickable and non-clickable objects becomes harder and harder with each version of macOS.
Mail Archiver can export your emails into mbox format. Mail is very picky what it imports. Eml files work better than mbox files. Postbox only imports files with a special structure. Outlook doesn't import mbox or eml files at all.
But you can easily upload your emails to an IMAP account.