In Mail direkt wohl nicht, aber fast: TNEF's Enough for macOS
http://www.joshjacob.com/mac-development/tnef.php
Open File
TNEF's Enough opens winmail.dat files to show you what's inside them. Assuming you have the winmail.dat file in an email, here are a couple ways to open it:
Save the file out of your email program. Choose Open... from TNEF's Enough File menu and locate the file.
Save the file out of your email program. Drag the file to the TNEF's Enough icon in the Dock.
Simply drag the attachment from your email program to the TNEF's Enough icon in the Dock.
Open File
TNEF's Enough opens winmail.dat files to show you what's inside them. Assuming you have the winmail.dat file in an email, here are a couple ways to open it:
Save the file out of your email program. Choose Open... from TNEF's Enough File menu and locate the file.
Save the file out of your email program. Drag the file to the TNEF's Enough icon in the Dock.
Simply drag the attachment from your email program to the TNEF's Enough icon in the Dock.