Mail starts on every boot in macOS Catalina
Solution 1:
Which 'Open at Login' option are you referring to specifically?
Things to check:
- System Preferences > Users & Groups > [your user account] > Login Items tab
If mail has been added their remove it.
- When you log out, restart or shutdown, you should get a "Reopen windows when logging back in" option. Make sure that is either unchecked, or manually quit Mail before your log out, restart or shutdown.