Letter Opener in macOS Mail
BackLink: 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.