Thanks, but that doesn’t appear in my Systems Settings, Login Items & Extensions. I’m running an older version 11.7 Big Sur. I can’t find a place anywhere with anything like that.
Try opening Systems Settings (or whatever it was called back then… It has been a while when I ran, and could run, Big Sur.) Then in the search field on the top left, enter
login items. That ought to open the page you need to get to, and should list all of the items that get loaded on startup. Look for
Mail.app, select it, and click on the minus sign at the bottom of the list.