MIME, or Multipurpose Internet Mail Extensions, refers to an official Internet standard that defines how messages must be formatted so that they can be exchanged among various email systems. The headers that define MIME messages are defined by RFC 2045, and the extensions that permit non-US-ASCII text data in Internet mail header fields is defined by RFC 207. Finally the MIME message formats and acknowledgements are defined by RFC 2049.

MIME is a very flexible format that permits virtually any type of file or document message type that can include text, images, audio, video, or other data. MIME uses base64 as an encoding procedure to ensure protection for non-text message. Ironically, it achieves this encoding by coding non-text messages as text.

How it Works

MIME type comprises a combination of type and subtype, and the charset of a text type reveals the encoding. Internet protocols such as HTTP use the content-type header and MIME type registry. MIME enables messages to have a tree structure, and it offers many features that are considered essential for modern email usage:

  • Support for character sets other than ASCII, required for sending email in languages other than English.
  • A content type labeling system, which allows multimedia content to be handled intelligently by computer programs.
  • Support for content in email messages that is not text, which allows email to contain multimedia content including images, audio, office documents, and more.
  • Support for compound documents, which allows a single email message to contain multiple parts (multiple images, file attachments, and so on).

Basic Usage

The MIME format is very similar to the format of information that is exchanged between a Web browser and its Web server. This related format is specified as part of the Hypertext Transfer Protocol (HTTP). Virtually all human-written Internet e-mail and a fairly large proportion of automated e-mail are transmitted via SMTP in MIME format.

Internet email has come a long way since RFC 822 was published in 1982. Today, all the mainstream email programs are fully compatible with the MIME standard for email, which allows for some advanced features and interoperability. The user-visible features that depend on MIME include styled text, text in non-Roman alphabets, file attachments, and multimedia content.

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