MIME Encoding Methods

To send non-ASCII data in MIME, it is necessary that it be encoded. The Content-Transfer-Encoding header is used to specify how a MIME message or body part has been encoded, so that it can be decoded by its recipient. The following types of encoding are defined:

• 7bit: This indicates that the message is already in ASCII form compatible with RFC 822. It is the default and is what is assumed if no Content-Transfer-Encoding header is present.
  
  
• 8bit / binary: These synonymous values mean the message has been encoded directly in 8-bit binary form. Yes, I did just say that this would violate the rules of RFC 822. These options appear to have been included to support future mechanisms for transporting binary data directly. RFC 1652 describes an SMTP extension that discusses this in part: SMTP Service Extension for 8bit-MIMEtransport (sic, there is no space between “MIME” and “transport”). However, the standard is clear that this still does not allow the transfer of raw binary data using SMTP and RFC 822.
  
  
• quoted-printable: This is a special encoding that is used when most of the data is ASCII text, but when it contains certain violations of the rules of RFC 822. These illegal sections are converted using special encoding rules so the data as a whole is consistent with RFC 822.
  
  
• base64: An encoding used to allow arbitrary binary data to be represented in ASCII form. The data is then sent as ASCII and decoded back into binary form by the recipient.

The quoted-printable and base64 encodings are the most interesting ones, because they are what allow non-RFC-822 data to be sent using RFC 822.

This encoding method is used when the message is “mostly” ASCII; only the problem bytes are encoded. The result is that RFC 822 compatibility is achieved while maintaining most of the data as regular text so it can still be easily understood by a human.

An example of where this would be letters with tildes or accents, such as those used in French or Spanish. Another would be a text message formed using an editor that inserts carriage return characters in the middle of a line. Most of the message is still text. The quoted-printable encoding can be used here, with the carriage return characters represented as “=0D” (the hexadecimal value of the character prepended by an equal sign). RFC 2046 contains more details on how this is done.