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.