Download Letter Opener on the Mac App Store
Click here to open the Mac App Store. Next click “Get” and then “Install App” to download and install Letter Opener on your system.
Open Letter Opener for macOS
After the installation is complete, open Launchpad, type “Letter Opener” in the search field and then click the App Icon.
Open a Winmail.dat, MSG or XPS file
Just double-click the document icon as shown below:
You can also open a file by dragging it onto the Letter Opener dock-icon.