EMAIL file extension is used by Microsoft Outlook Express and is the format used to store email messages when exported. File extension EML and File extension MSG are also valid extensions for this type of file and generally the files are treated as plain text files although they can also contain macros and email attachments, and as such should be treated with caution. The specific MIME type for these files is message/rfc822. This email message format is a standard and open format and is described at length in RFC 822: Standard for ARPA Internet Text Messages available on-line (from http://www.w3.org/Protocols/rfc822 for example).
A number of high profile computer worms have used this file type to send copies of themselves across the net via email. This was one of the many methods that the famous W32/Nimda worm used, for example. An infection could be determined by the presence of several EML files spanning multiple folders. These messages took advantage of a vulnerability in the way mail clients display HTML emails via an Internet Explorer control in order to run an attached executable file embedded in the email file itself. If you receive a file with the extension EML or EMAIL you should always run a virus scan on it before opening it.
There are several email clients that can open EML file, as well as dedicated mail file readers such as MSGViewer Pro from Encryptomatic which can open both EML and MSG files. They can also be viewed in Internet Explorer if the file extension is changed to MHT. Newer versions of Outlook and Outlook Express disable the opening of EML files by default due to the security risks associated with these files, so you may have to tweak the settings to get them to open.
For more general information about files, file extensions and registry you can read one of the following articles:
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