MIME, which stands for Multipurpose Internet Mail Extensions, is an Internet standard that extends the email protocol to support a range of elements, like multi-part message bodies, text from character sets beyond ASCII, non-text attachments to messages, and non-ASCII message header information. Originally designed for the SMTP protocol, modern MIME use extends past merely describing the content of email, and is often used to describe general web content and for rich media storage in certain commercial software products.

Essentially all email composed by people and a large percentage of automated email is sent by SMTP in MIME format. In fact, Internet-based email is now so closely associated with the MIME and SMTP standards that is often referred to as SMTP/MIME email.

MIME header types include MIME-version, which is used to indicate that the message is MIME-formatted; Content-Type, which specifies the media type of the message content and consists of a type and subtype; Content-Disposition, which specifies the style of presentation (inline or attachment); and Content-Transfer-Encoding, which specifies whether or not binary-to-text encoding has been used, and if so, which variety, and if not, provides a label that describes the format of the content.