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)

MIME Content-Transfer-Encoding Header and Encoding Methods 1 2 TCP/IP Electronic Mail Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)

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:
  • “B”: This indicates base64 encoding, as described in the previous topic.
    
    
  • “Q”: This indicates quoted-printable encoding, again as in the prior discussion.
  
  
  
• <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.

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

MIME Content-Transfer-Encoding Header and Encoding Methods 1 2 TCP/IP Electronic Mail Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)