MIME is an Internet standard that extends the format of e-mail to support:

* text in character sets other than US-ASCII;
* non-text attachments;
* multi-part message bodies; and
* header information in non-ASCII character sets.

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MIME's use, however, has grown beyond describing the content of e-mail to describing content type in general.

Virtually all human-written Internet e-mail and a fairly large proportion of automated e-mail is transmitted via SMTP in MIME format. Internet e-mail is so closely associated with the SMTP and MIME standards that it is sometimes called SMTP/MIME e-mail.

The content types defined by MIME standards are also of importance outside of e-mail, such as in communication protocols like HTTP for the World Wide Web. HTTP requires that data be transmitted in the context of e-mail-like messages, even though the data may not actually be e-mail.

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MIME defines mechanisms for sending other kinds of information in e-mail. These include text in languages other than English using character encodings other than ASCII, and 8-bit binary content such as files containing images, sounds, movies, and computer programs. MIME is also a fundamental component of communication protocols such as HTTP, which requires that data be transmitted in the context of e-mail-like messages even though the data might not fit this context. Mapping messages into and out of MIME format is typically done automatically by an e-mail client or by mail servers when sending or receiving Internet (SMTP/MIME) e-mail.

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The basic format of Internet e-mail is defined in RFC 2822, which is an updated version of RFC 822. These standards specify the familiar formats for text e-mail headers and body and rules pertaining to commonly used header fields such as "To:", "Subject:", "From:", and "Date:". MIME defines a collection of e-mail headers for specifying additional attributes of a message including content type, and defines a set of transfer encodings which can be used to represent 8-bit binary data using characters from the 7-bit ASCII character set. MIME also specifies rules for encoding non-ASCII characters in e-mail message headers, such as "Subject:", allowing these header fields to contain non-English characters.

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J. Jeyaseelan
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MIME is extensible. Its definition includes a method to register new content types and other MIME attribute values.

The goals of the MIME definition included requiring no changes to extant e-mail servers, and allowing plain text e-mail to function in both directions with extant clients. These goals were achieved by using additional RFC 822-style headers for all MIME message attributes and by making the MIME headers optional with default values ensuring a non-MIME message is interpreted correctly by a MIME-capable client. A simple MIME text message is therefore likely to be interpreted correctly by a non-MIME client although it has e-mail headers the non-MIME client won't know how to interpret. Similarly, if the quoted printable transfer encoding (see below) is used, the ASCII part of the message will be intelligible to users with non-MIME clients.

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J. Jeyaseelan
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