How to convert a winmail.dat file (on a Mac)
On a Mac, to convert a winmail.dat file into the original format is to download the free program called “TNEF’s enough”. Alternatively, there is a paid OS X Mail plugin available called “Letter opener” which will automatically decode the attachment still in Mail.
Gmail, via the web interface, also decodes winmail.dat files. Outlook 2010 on Windows converts them automatically, though as of this writing Outlook 2011 on the Mac does not. Thunderbird has a plugin, “Lookout” which also converts the file.