Although some MIME content types can be
converted to 7 bits, not all types should be. Content types are
defined by the Content-Type: header (Content-Type:). For example, the type
multipart/ should not be converted, whereas its
component boundary-separated parts probably should be. Conversion is
done by encoding with either quoted-printable or base64 (EightBitMode).
Beginning with V8.7 sendmail, types that should
not be encoded are those defined as members of the class
$=n. When sendmail first
starts to run, it defines the following list of values for class
As of V8.10, no other useful values exist for this class.
Note that a type in class $=n can still be
prevented from being encoded based on the considerations imposed by
class $=e. Also note that the actual encoding can
be restricted to quoted-printable by use of the class