Hey! Thanks for reporting this—when you check/uncheck the box, Mailspring creates and removes ~/Library/LaunchAgents/com.mailspring.plist. It looks like right now there's no user feedback if that fails for some reason. It also looks like if you go to System Preferences and configure Mailspring to launch at start from the Users & Groups preference pane under Login Items, that creates a /separate/ preference in another locaiton which could separately launch Mailspring even if you unchecked the box within the app.

This is a bit of a mess—I'll see if we can revisit this and clean up the MacOS implementation to do the same thing as adding the app as a "Login Item" in system preferences. In the meantime, I'll make the app present an error message if removing it's LaunchAgent plist fails. You may be having a permissions issue of some sort and it's just not letting you know it's unable to delete that file.

Thanks!