The Reverting Preference
On macOS, Apple Mail is established as the default application for handling all email communication. When you click an email address link (`mailto:`) on a website, Apple Mail automatically launches. If you prefer to use a third-party application like Microsoft Outlook or Spark, you can change the default client. However, due to a quirk in how macOS handles application updates, or sometimes simply after a system reboot, macOS might automatically change the default mail client right back to Apple Mail. This forces you to constantly re-apply your preference and interrupts your workflow when the wrong app keeps launching.
Locking the Mail Client
To ensure your third-party email application remains the permanent default and stop macOS from automatically changing it back to Apple Mail, you must set the preference from within Apple Mail itself, rather than System Settings.
Counterintuitively, the master setting for this is hidden inside the app you are trying to avoid. Open the Apple Mail application (you do not need to add an email account to it if prompted, just get to the main menu). Click on Mail in the top-left corner of the menu bar and select Settings (or Preferences). Ensure you are on the General tab. Look for the very first dropdown menu at the top, labelled Default email reader. Click this dropdown and select your preferred application (e.g., Microsoft Outlook). Once selected, close the Mail settings window and quit Apple Mail. macOS will now respect this setting permanently, even across most system updates.