Microsoft Outlook and other popular email clients use EML (MIME RFC 822 standard) as a standard format. EML is the abbreviation for electronic mail or email. Similarly, it is a file extension for an email message saved to a file in the Internet Message Format protocol for electronic mail messages. The EML files usually contain plain ASCII text for the main message body, headers and hyperlinks, and attachments. However, there is one threat associated with the EML format is that it can be a phishing email and the attachment could either: Infect your machine with malware if you open the attachment.