Multipurpose Internet Mail Extensions (MIME)

Definition of Multipurpose Internet Mail Extensions (MIME) in The Network Encyclopedia.

What is MIME (Multipurpose Internet Mail Extensions)?

A widely used Internet standard for encoding binary files to send them as e-mail attachments over the Internet.

How It Works

Traditional e-mail sent over the Internet using Simple Mail Transfer Protocol (SMTP) as specified by Request for Comments (RFC) 822 defines messages as consisting of a header and a body part, both of which are encoded using 7-bit ASCII text encoding. The header of an SMTP message consists of a series of field/value pairs that are structured so that the message can be delivered to its intended recipient. The body is unstructured text and contains the actual message.

Multipurpose Internet Mail Extensions (MIME) defines five additional extensions to SMTP message headers, supports multipart messages with more than two parts, and allows the encoding of 8-bit binary data such as image files so that they can be sent using SMTP. The encoding method for translating binary information used by MIME, Base64 Encoding, essentially provides a mechanism for translating nontext information into text characters.

The MIME extensions are implemented as fields within the e-mail message header. These fields define the following:

  • Content type
  • Content transfer encoding method
  • MIME version number
  • Content ID (optional)
  • Content description (optional)

Microsoft Exchange Server uses MIME to encode messages in foreign character sets and for sending Rich Text Format (RTF) messages that include formatting such as italics and underlining.