The EML format is the standard format used by many email programs. EML files are created to comply with industry standard RFC 5322. As such, EML files can be used with most email clients, servers, and applications.
The EML format is used to store email messages on a file system. Each message is stored in a single file (unlike MBOX, which concatenates all the messages from a folder into one file), and attachments are included as MIME content in the file.
Besides the Microsoft Outlook Express, EML files can be opened using most email clients such as Microsoft Outlook, Microsoft Entourage, Mozilla Thunderbird, Apple Mail, or IncrediMail.
Since EML files are plain text and formatted much like MHT (MIME HTML) files, they can also be opened directly in Internet Explorer, Mozilla Firefox, and Opera, by first changing the file extension from .eml to .mht. It is also possible to view EML files using notepad or any other text editor.
Windows 8, however, does not natively support EML in the built-in Mail application, so an EML Viewer was developed to support reading EML files.