The Unwanted Inbox
The default Apple Mail app is a core part of macOS. However, many users prefer third-party email clients or simply use webmail in their browser. If you fall into this category, it is incredibly annoying when the Apple Mail app insists on automatically launching and opening its main window every single time you start up or log into your Mac. This adds unnecessary time to your boot process and clutters your desktop with an application you have no intention of using.
Removing Mail from Startup
To keep your desktop clean on boot and stop macOS from automatically opening the Apple Mail app when you log in, you must check your Login Items and the app’s Dock settings.
First, check your system-wide startup applications. Open System Settings (click the Apple logo in the top-left). In the left sidebar, click General, then click Login Items on the right. Look at the “Open at Login” list. If you see “Mail” listed here, click on it to highlight it, then click the small minus (-) button below the list to remove it. If Mail is not in that list, the issue might be a Dock preference. If the Mail icon is in your Dock at the bottom of the screen, right-click (or Control-click) the Mail icon. Hover over Options in the context menu. If there is a checkmark next to Open at Login, click it to uncheck it. Your Mac will now boot without the Mail app interrupting you.