EML or Electronic mail is a file format primarily associated with Microsoft Corporation “Outlook Express” and is used by other email clients as well. It is a file extension (.eml) for e-mail message saved to file in MIME RFC 822 standard format. Basic purpose of EML file is storing email messages as a plain text file in standard file structure. It can contain plain ASCII text for headers, main message body, attachments and hyperlinks as well. Header comprises of e-mail address of sender and recipient, subject, time & date stamp of message.
EML file extension for email files also referred as RFC-822 file format was created in 1982 as an enhancement in RFC-733 through establishment of ARPA standards for emerging use of emails. Its purpose was also to simplify the address standards which were not uniformly recognized in software business.
Microsoft Corporation also manufactured series of COM modules which was included in Outlook Express email application’s development as well. These modules were part of Microsoft Office suite products. RFC is a standard format but Microsoft create PST file format to store emails as a highly structured database. But it restricted the access of emails without presence of proprietary application module. This file format was created to store email files offline with a functionality of forwarding email messages. Eventually this created an issue for non-Microsoft email clients when received from Outlook application. Since then, many other email applications created emails as .eml files saving the file in MIME RFC-822 format. Primarily associated with Outlook Express EML file format is also part of other email clients like Thunderbird, Apple Mail, etc. Outlook Express successor email client Windows Live Mail also use EML file format to save emails.
Being a plaintext and analogous to MHT file format, EML can be opened in a webbrowser by changing the extension to .mht. However, Windows 8 built-in Mail application does not support EML.