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 $=n:

multipart/signed

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 $=q.