Since Domino 6.5.6, 7.0.2 and all 8.x releases there has been a new Domino server based notes.ini setting to extract winmail.dat files and "convert" them to readable format. Simply add the following setting to the server and restart the server:

TNEFEnableConversion=1

Note, if you are using Domino 7.0.2 make sure you have at least FP1 installed as there is a frequent crash condition in the base 7.0.2 release code.

For further details see IBM Technote 1093342.