The EightBitMode
(8) option (EightBitMode)
determines when and how 8-bit data will be encoded into a 7-bit
format. Ordinarily, the decision to use quoted-printable as opposed
to base64 is made by examining the input stream and choosing
quoted-printable if less than 1/8 of the first 4 kilobytes of data
has the high bit set. Otherwise, encoding is with base64.
Beginning with V8.8, sendmail offers the class
$=q as the means to force the selection of
quoted-printable. Just before scanning the input data,
sendmail extracts the
type and
subtype from the
Content-Type: header (Content-Type:):
Content-Type: type/subtype; ...
If the type is in the class
$=q, the body will definitely be encoded with
quoted-printable if encoding occurs. Also, if a concatenation of
type, a slash (/), and
subtype is in class
$=q, the body will definitely be encoded with
quoted-printable.
When sendmail first begins to run, class
$=q is empty. A reasonable value in most countries
might be text/plain (although probably not in
countries that use 16-bit characters, such as China). Other values
for this class might be text or
text/html.
