EML is a shortened version for the word email. It is the standard format used by Microsoft Outlook Express, Thunderbird, and Windows Live Mail.
It was developed to comply with the standard RFC 5322, so it can be used with most email clients and applications. However, there is no specification available because il is commonly considered to be an extension of IMF (Internet Message Format) as defined in RFC 5322.
EML files are usually 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.
They store each message as a single file, and attachments can either be included as MIME content in the message or written off as a separate file, referenced from a marker in the EML file.
MSG (Microsoft Outlook Item) is the extension supported by Microsoft Outlook in the following products: Microsoft Exchange Server 2003-2013 and Microsoft Office Outlook 2003-2013. Its specification is covered by the Microsoft Interoperability Program.
The .msg file format can store a single Message object (like an email, an appointment, a contact, a task, and more), in a file. Attachment objects, are also present in the file.