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;
        boundary="boundary"