The Stubborn Client
macOS comes pre-installed with Apple Mail, which is set as the default email client. Many users prefer third-party applications like Microsoft Outlook, Spark, or Thunderbird. You can easily change your default mail app, but a frustrating bug sometimes occurs: after a major macOS update, or after briefly opening Apple Mail to check an archived message, the operating system quietly resets your preference, hijacking email links (mailto: links on websites) and forcing them to open in Apple Mail once again, completely bypassing your preferred third-party client.
Enforcing Your Preference
To establish your preferred email client permanently and stop macOS from automatically changing the default mail app back to Apple Mail, you must adjust a deeply hidden setting within the Apple Mail application itself.
Counterintuitively, you cannot change this in the main macOS System Settings. You must open the native Apple Mail application (even if you hate it). Once Apple Mail is open, look at the very top Menu Bar on your screen. Click on Mail and select Settings… (or Preferences). A dialogue box will appear. Ensure you are on the General tab at the very top. Look for the dropdown menu explicitly labelled Default email reader. Click this dropdown and select your preferred application (e.g., Microsoft Outlook). Once selected, you can completely close Apple Mail. By setting the preference from *inside* Apple’s own app, it writes a permanent configuration flag that the operating system is far less likely to overwrite during subsequent system updates.