Letter Opener in macOS Mail Go back Go back Link: https://winmail.help/t/manage-plugins 1. In Mail go to Mail > Preferences … 2. In the following window click "Manage Plug-ins …" 3. Enable the checkbox by Letter Opener and click "Apply and Restart Mail" Troubleshoot Installer can’t be opened because it was not downloaded from the App Store. Winmail.dat file is not converted.