I had the same problem and this worked for me
From: How to enable Windows Mail app to operate in Windows 7 - TechSpot OpenBoards
* Manually edit the registry using the following steps.
* Click: Start ==> All Programs ==> Accessories ==> Run ==> Type in: regedit ==> OK
* Open: HKEY_CURRENT_USER ==> Software ==> Classes ==> eml_auto_file ==> shell ==> open ==> command
Note: If the "eml_auto_file" option is missing, it will normally not be necessary to do this edit. This key appears to be added when Windows Live Mail is installed. If Windows Live Mail is never installed it will not be there.
* Highlight: Command ==> In the right panel double click: Default ==> Copy and paste the following key into "Value Data":
"C:\Program Files\Windows Mail\winmail.exe" /eml:%1
* Save and exit.