I'm also having the same problem that I cannot open .eml attachments in WM using Windows 7.
I have tried the fixes in this thread .eml attachments won't open In windows mail in 'Seven
without much luck.
I don't have the registry key: HKEY_CURRENT_USER\Software\Classes\eml_auto_file\shell\open\command
I also tried downloaded mailclient.reg fix. Windows Mail doesn't show up in my Default Programs folder.
In Windows Mail, I have been into Tool>Options>General and changed it to Make this application the default Mail Handler.
I saved an .eml message to the desktop, did the file association to open with WM. All it does it opens the WM client and not the .eml file.
What did I miss?