Filename extensions

Upon reception of e-mail messages, e-mail client applications save message in operating system files in the filesystem. Some clients save individual messages as separate files, while others use various database formats, often proprietary, for collective storage. A historical standard of storage is the mbox format. The specific format used is often indicated by special filename extensions:

eml
Used by many e-mail clients including Microsoft Outlook Express, Windows Mail and Mozilla Thunderbird.[31] The files are plain text in MIME format, containing the e-mail header as well as the message contents and attachments in one or more of several formats.
emlx
Used by Apple Mail.
msg
Used by Microsoft Office Outlook.
mbx
Used by Opera Mail, KMail, and Apple Mail based on the mbox format.

Some applications (like Apple Mail) also encode attachments into messages for searching while also producing a physical copy of the files on a disk. Others separate attachments from messages by depositing them into designated folders on disk.