The MIME specification provides a mechanism to label the character set used in a plain text message. Specifically, a charset= parameter can be specified as part of the Content-type: header line. Various character set names are defined in MIME, including US-ASCII (the default), ISO-8859-1, ISO-8859-2, and so on.
Some existing systems and user agents do not provide a mechanism for generating these character set labels; as a result, some plain text messages may not be properly labeled. The charset7 and charset8 channel keywords provide a per-channel mechanism to specify character set names to be inserted into message headers. Each keyword requires a single argument giving the character set name. The names are not checked for validity.
Note - Character set conversion can only be done on character sets specified in the character set definition file charsets.txt found in the IMTA table directory,
/imta/table/charsets.txt. The names defined in this file should be used if possible.
The charset7 character set name is used if the message contains only seven-bit characters; charset8 is used if eight-bit data is found in the message. If the appropriate keyword is not specified, no character set name is inserted into the Content-type: header lines.
These character set specifications never override existing labels; that is, they have no effect if a message already has a character set label or is of a type other than text. It is usually appropriate to label IMTA local channels as follows:
If there is no Content-type header in the message, it is added. This keyword also adds the MIME-version: header if it is missing.