How to export an email from Mail application?
Actually that functionality is beyond simple. Drag the email from the Mail.app window to the desktop. When you reach the desktop the pointer then contains acircular green "+"
Drop the email there and you now have a single message as an .eml file
Harder way! If you know the contents of the mail search for it in ~/Library/Mail, and go to path like
~/Library/Mail/V6/<some-key>/INBOX.mbox/key/Data/2/5/Messages/52377.emlx
V6 may be different for other versions of macOS or Mail. For partial emails, remote content might be needed to download first.