mrbinky3000 gets credit for bringing the following reghack to my attention.

This reghack will allow you to open *.eml attachments (OE/Mail/Live mail messages) in Outlook's message form (if you want to reply to the message, use Alt+S to send if the send button is missing).

Warning: before editing the registry you should export the keys you are planning to edit and if you start having problems with Outlook, restore the keys. That said, I haven’t had any problems with this hack.

If you have Live Mail installed, go to

If Outlook express, go to:

Change the key to the following line, exactly as shown below, except you'll use the correct path for your version of Outlook:

Ie, Outlook 2007 on 64x Vista or Windows is

Outlook 2003 on 32bit Windows:

For years, I changed the association for eml (and nws) to notepad so email attachments or files on my hard drive would open outside of OE. (Before that, I changed the file extension to txt or html.) The advantage of using this method is the headers are hidden; the disadvantage is Outlook needs to be open.

Complete instructions and screenshots:
Open .eml Files in Outlook
http://www.slipstick.com/mail1/open_eml.htm