Introduction
The basic Internet email transmission protocol, SMTP, supports only 7-bit ASCII characters (see also 8BITMIME). This effectively limits Internet email to messages which, when transmitted, include only the characters sufficient for writing a small number of languages, primarily English. Other languages based on the Latin alphabet typically include diacritics and are not supported in 7-bit ASCII, meaning text in these languages cannot be correctly represented in basic email.
MIME defines mechanisms for sending other kinds of information in email. 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. Parts of MIME are also reused in communication protocols such as HTTP, which requires that data be transmitted in the context of email-like messages even though the data might not (and usually doesn't) actually have anything to do with email, and the message body can actually be binary. Mapping messages into and out of MIME format is typically done automatically by an email client or by mail servers when sending or receiving Internet (SMTP/MIME) email.
The basic format of Internet email is defined in RFC 5322, which is an updated version of RFC 2822 and RFC 822. These standards specify the familiar formats for text email headers and body and rules pertaining to commonly used header fields such as "To:", "Subject:", "From:", and "Date:". MIME defines a collection of email 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 email message headers, such as "Subject:", allowing these header fields to contain non-English characters.
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 existing email servers and allowing plain text email to function in both directions with existing 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 even if it has email headers which 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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