EML is the IETF-standard email message format (.eml extension), plain-text headers (From, To, Subject, Date), a blank line, then the body. MIME multipart structure handles HTML email bodies and attachments. Most email clients (Outlook, Apple Mail, Thunderbird) save individual messages as EML when you drag them out of the inbox.
How to open
Outlook, Apple Mail, Thunderbird, Gmail's web import. Plain text in any editor for header inspection.