Problem : When you attempt to open an .eml attachment in Outlook Express, either the file may not open or you may receive an error message that states that there is no program associated with this file type.

Cause : This behavior can occur if an installed third-party e-mail program has an associated .eml file type.

Solution 1:
1. Quit Outlook Express.
2. Click on the Start
3. Click Run.
4. In the Open box, type "msimn /reg", and then click OK.

Solution 2:
1. On the desktop, double-click My Computer.
2. On the Tools menu, click Folder Options.
3. Click the File Types tab up on the top.
4. Under “Registered file types,” click “EML Outlook Express Mail Message”, and then click Advanced.
5. Click Open, and then click Edit.
6. Type the following single line in the “Application used to perform action” box:
"C:\Program Files\Outlook Express\MSIMN.EXE" /eml:%1
7. Click OK, click OK again, and then click Close.