Occasionally I save an email message from Thunderbird to a file, and when I do that the file has an .eml extension. For those new to this concept, the .eml file extension is usually applied to files in the MIME RFC 822 standard format used by email applications. You can open that .eml file using Thunderbird (which is available on Mac OS X, Windows, and Linux) or using Mail.app on Mac OS X, and I’m sure using many other applications that adhere to the standards. I can even open .eml files in a text editor and easily read the text/plain portion of a multi-part email message.
Unfortunately I recently came across some emails with a .msg extension. It seems that Microsoft Outlook saves emails files to disk as .msg files, which is probably not the MIME RFC 822 standard format, and is some weird format you can’t easily read without Outlook… (It’s actually based on the “Compound File Binary Format” and require a MAPI-aware application to view them.)
Luckily MsgViewer is an application (well, a JAR file) which (as long as you’ve got Java installed) can open these .msg files. You can grab MsgViewer from SourceForge.
And seriously, why does Microsoft do this kind of stuff??