Kernel EML Viewer (ver 11.05.01) Abandoned Freeware EML is a file extension for an email message saved to a file (typically for archiving purposes) in the MIME RFC 822 format used by Microsoft Outlook Express as well as some other email programs. EML files are plain text files, but they can contain HTML codes and attachments (emailed binary files are always encoded to plain text) which can be decoded with an intelligent mail viewer.