Note: Outlook 2007 SP2 and Outlook 2010 can open eml files directly without using this hack.
When you receive an .eml attachment in a message and open it, it opens in Outlook Express or Windows Live Mail, not in Outlook. This is normal since eml is not an Outlook file type.
However, .eml files are simple text files and can be opened in Notepad or a web browser, either by changing the extension from eml to txt or HTML, or by changing the file association to Notepad. I've used this method for years to avoid opening OE when I open eml files in email or on my hard drive.
The registry hack below opens the *.eml files as an Outlook message. The Reply and Forward buttons work but the resulting message may be missing the Send button. If so, use Alt+S to send instead.
If you use both Outlook and OE or WLM, *.eml attachments on messages within OE/LM will open in Outlook and replies will be sent using Outlook.
This method comes from mrbinky3000 and was tested on Windows 7 64-bit, I tested it on WindowsXP with Outlook Express. The steps below assume you are using Outlook 2007 and Windows7 64-bit, and have Live Mail installed. You will use the correct path for your version of Outlook. Keys for Outlook Express and Vista Mail are listed in the next section.
This will open all .eml files that are in the file system using Outlook, including *.eml attachments on messages in OE/WLM. Any replies will be sent using Outlook.
1) Type regedit in the Search field on the Start menu or press Windows Key+ R and enter it in the Run command.
2) Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Microsoft Internet Mail Message WLMail\shell\open\command
3) Change the command line (here its "C:\Program Files (x86)\Windows Live\Mail\wlmail.exe" /eml:%1) to the following line, exactly as shown below but correcting the path to Outlook.exe to match the path on your computer.
"C:\Program Files (x86)\Microsoft Office\Office12\OUTLOOK.EXE" /eml "%1"
This way, windows thinks that "Windows Live Mail" is still the default program for opening .eml files. However, we changed the registry so that instead of opening Windows Live Mail, we are opening OUTLOOK.EXE. What I mean is that if you opened the control panel, clicked on Default Programs and associated .eml files with Windows Live Mail, we've tricked out the registry so that it will open outlook instead–even though Windows still says it is associated with Windows Live.
This hack works with Outlook Express and Mail and versions of Outlook. You need to identify the correct registry key to change and use the correct path to your version of Outlook.
The key for Outlook Express is:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Microsoft Internet Mail Message\shell\open\command
If Outlook 2010 "Click2Run" is installed, it may take control of the *.eml, even if you used the path to an older version of Outlook in the registry key.
The reply and forward buttons are 'live' on eml's opened in Outlook, but there may not be a Send button. Saving to drafts and reopening does not show a Send button, however Alt+S does send the message.
How do I get Outlook 2007 to read .eml files and attachments? (Microsoft Answers forum)