transfer level
**************

   Contents inserted in a message are represented by 7bit (*Note
(tm-en)7bit::), 8bit (See (tm-en)8bit) or binary (*Note
(tm-en)binary::).

   If a message is translated by 7bit-through MTA (See (tm-en)MTA),
there is no need to encode 7bit data, but 8bit and binary data must be
encoded to 7bit data.

   Similarly, if a message is translated by 8bit-through MTA, there is
no need to encode 7bit or 8bit data, but binary data must be encoded to
7bit or 8bit data.

*[Memo]*
     EBCDIC MTA breaks 7bit data, so in this case, 7bit data must be
     encoded by base64.  But I don't know EBCDIC. (^_^;

     Similarly, I wish ASCII-printable only MTA and code-conversion MTA
     disappeared. (^_^;

     Maybe there are binary-through MTA, but I think it is not major.

   *transfer level* represents how range data is available.  tm-edit
has a variable `mime-editor/transfer-level' to represent transfer level.

 - Variable: mime-editor/transfer-level
     transfer level.

     If transfer level of a data is over it, a data is encoded to 7bit.

     Currently, 7 or 8 is available.  Default value is 7.

     In extension plan, EBCDIC will be 5, ASCII printable only will be
     6, binary will be 9.  But it will not be implemented.

*[Memo]*
     transfer level is only for body, not for header (See header).
     RFC 1521 extends RFC 822 (See (tm-en)RFC 822) to use 8bit data
     in body, but it requires to use us-ascii (See (tm-en)us-ascii)
     in header.