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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
                     9  TCP/IP Electronic Mail Message Formats and Message Processing: RFC 822 and MIME
                          9  TCP/IP Enhanced Electronic Mail Message Format: Multipurpose Internet Mail Extensions (MIME)

Previous Topic/Section
MIME Content-Transfer-Encoding Header and Encoding Methods
Previous Page
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1
2
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TCP/IP Electronic Mail Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)
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MIME Extension for Non-ASCII Mail Message Headers
(Page 2 of 2)

MIME Encoded-Word Syntax

In the MIME non-ASCII header technique, the value of a regular header is replaced by a MIME encoded-word that has the following syntax:

=?<charset>?<encoding>?<encoded-text>?=

The strings “=?” and “?=” are used to bracket the non-ASCII header, which flags it as a MIME encoded header to the recipient's e-mail client. The other elements, separated by “?”, indicate how the non-ASCII text is encoded:

  • <charset>: The character set used, such as “iso-8859-1”.

  • <encoding>: Two different encoding types are defined, each represented by a single letter for brevity:

  • <encoded-text>: The non-ASCII text that has been encoded as ASCII using the encoding type indicated.

As you can see, this method is analogous to how a non-ASCII message body or body part would be encoded, but the information about the encoding has been condensed so everything can fit in a single header line. The “<charset>” parameter is somewhat analogous to the Content-Type header for a message body, but since headers can only contain text, it specifies what kind of text it is. The “<encoding>” parameter is clearly equivalent to the Content-Transfer-Encoding header.

Example Non-ASCII MIME Header

Here's an example of a non-ASCII header, using the GB2312 character set (for Chinese characters) and base64 encoding:

Subject: =?GB2312?B?u7bTrbLOvNPDwLn61bm74Q==?=

I hope that doesn't say anything inappropriate; I took it from a piece of spam e-mail I received once! J

Key Concept: In addition to its many functions for encoding a variety of data in e-mail message bodies, MIME provides a feature that allows non-ASCII information to be placed into e-mail headers. This is done by encoding the data using either quoted-printable or base64 encoding, and then using a special format for the header value that specifies its encoding and character set. This technique is especially useful for e-mail sent in languages that cannot be represented easily in standard ASCII, such as many Asian languages.


 


Previous Topic/Section
MIME Content-Transfer-Encoding Header and Encoding Methods
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Pages in Current Topic/Section
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2
Next Page
TCP/IP Electronic Mail Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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