MIME Capabilities

MIME defines a message format that allows for:

• Textual message bodies in character sets other than US-ASCII.
• Non-textual message bodies.
• Multipart message bodies.
• Textual header information in character sets other than US-ASCII.

MIME provides a mechanism for sending a variety of file types as a part of or along with regular mail messages. It uses the headers defined by RFC 822 and extensions to those header definitions. MIME does all this while following all the standard SMTP and RFC 822 mail rules. MIME messages can be constructed to transport mail over any mail transport system that is compliant with SMTP. MIME is able to transmit objects with varying ranges of complexity in a way that allows any MIME-compliant user agent (UA) to faithfully process them and hand them off to an appropriate application.

MIME accomplishes this simplifying and rebuilding of complex files by encoding a file (see Content-Transfer-Encoding) and transporting it as a message body, or a series of messages with component parts of the file. A MIME-compliant user agent (UA) on the receiving end can decode the message, presenting it to the reader or providing it to another application as a file. A UA that is not MIME-compliant will be able to process a MIME-encoded mail message, but may not be able to decode the message.

MIME allows for multipart mail messages, ensuring that the mail reader (UA) with even the most limited capabilities can still process the message. The multiple parts are arranged so that the parts requiring the least sophisticated UA are at the beginning of the message. In fact, most MIME UAs include courtesy text when constructing messages to give users of non-MIME UAs an indication of the message content. This courtesy text is inserted ahead of any MIME parts.