eml is the extension for a file used to hold email. It is used by MS Outlook and several other email programs.
 EML capabilities
An eml file can contain the email as well as attachments. There is some risk in opening attachments as they could be a virus. The formatting is similar to HTML. Internet Explorer, Firefox, or Opera can read an EML file by changing the extension to .MHT and then opening it.
Web document formatted using the MIME (Multipurpose Internet Mail Extensions) HTML or "MHTML" standard; encodes images and other media files referenced by a .HTML document into the same file the HTML is stored in; the data is encoded using .MIME encoding.
Most Web pages viewed in a Web browser only contain HTML and reference images and other media files that are stored in other directories on the server; because all the files are encoded into a single file in a MHTML document that has an extension of .MHT. MHT files are commonly used to archive Web pages.
Content-type: multipart/mixed; boundary="someMagicString" The general format of a multipart MIME message will be the following: To: firstname.lastname@example.org Subject: MIME test Content-type: multipart/mixed; boundary="theBoundaryString" --theBoundaryString Plain text message goes in this part. Notice that it has a blank line before it starts, meaning that this part has no additional headers. --theBoundaryString Content-Type: text/html Content-Transfer-Encoding: 7bit Content-Disposition: inline Content-Base: "http://somewebsite.com/" <body><font size=4>This</font> is a <i>test</i>. --theBoundaryString--
 For more information
MIME encode standard for HTML documents