Ordinarily, MIME mail is translated into SMTP format before it is encoded with base64. Specifically, the newline character that ends each line is converted into the SMTP carriage-return/linefeed form before being encoded. This adds time to the process, and extra size to the result, and for some forms of MIME mail this translation makes little sense. Video, for example, is not text-oriented, and so should not be treated like text (even though it will be encoded as text for transmission).

Beginning with V8.8, sendmail will skip converting newlines under certain conditions. Before deciding to convert, sendmail extracts the type and subtype from the Content-Type: header (Content-Type:):

Content-Type: type/subtype; ...

If the type is in the class $=b, newline conversion will be skipped. If a concatenation of type, a slash (/), and subtype is in class $=b, newline conversion will also be skipped.

Note that this class is not automatically available. To use it in this way, you need to define USE_B_CLASS when you compile sendmail.

If you define USE_B_CLASS, sendmail will automatically assign to class $=b the values application/octet-stream, image, audio, and video.