The Content-Type: header
describes the nature of the body of a mail message. In the absence of
such a header, the body is presumed to be composed of ASCII
characters that have their high (most significant) bits turned off.
One possible setting for this header might look like this:
Content-Type: text/plain; charset=ISO-8859-1
This header says that the body is plain text (i.e., contains no
markup language) and is represented in the ISO-8859-1 character set.
This header is usually created by the originating MUA. It should
never be declared in the configuration file of pre-V8.7 versions of
sendmail. Beginning with V8.7, the
charset for 8- to 7-bit MIME conversions can be
declared with the DefaultCharSet option (DefaultCharSet).
When bouncing mail, V8 sendmail creates a
MIME-compliant message and includes a
Content-Type: header such as this:
Content-Type: multipart/mixed; boundary="boundary"
If sendmail was compiled to include DSN support
(DSN), the Content-Type:
header will look like this:
Content-Type: multipart/report; report-type=delivery-status;