The F=7 delivery agent flag (F=7) determines whether MIME-encoded data should be converted from 8 to 7 bits. If the message is in 8-bit format and if it is going to a MIME-capable destination that requires 7-bit data, the message body will be converted to 7 bits by using either quoted-printable or base64 (EightBitMode).

Not all datatypes should be converted to 7 bits, however. The types that might possibly be converted are listed with the Content-Transfer-Encoding: header (Content-Transfer-Encoding:). One type that should not be converted, for example, is the quoted-printable type because it is already converted. Types that can be converted are 7bit, 8bit, and binary.

Beginning with V8.7 sendmail, the class $=e is used to determine whether a type will be encoded. Only those values listed in this class will be encoded. When sendmail first starts, it initializes the list of values in class $=e to be:

7bit 8bit binary

You can add types to this class, but you can never remove them.

Note that a type in class $=e can still be prevented from being encoded on the basis of the considerations imposed by class $=n. Also note that the actual encoding can be restricted to quoted-printable by use of the class $=q.