In OS X Lion, Apple has decided to drop support for exporting RTFD (i.e. RTF files with pictures embedded) from Mail. So if you want to export an e-mail with images, the only option is the newly added “raw message format”, which will give you an EML file. But what to do with this file?
According to Wikipedia, .eml files are:
plain text in MIME format, containing the email header as well as the message contents and attachments in one or more of several formats.
… web page archive format […] encoded as if it were an HTML e-mail message …
Strangely enough, Safari cannot open .MHT files. Through an add-on, it is also possible to store MAFF files. And then this kind of points to the fairly new EPUB standard.
Phew, too many formats.