MIME is documented in RFC2045, RFC2046, RFC2047, RFC2048, and RFC2049. The sendmail program cares about MIME only when bouncing messages and when determining how to convert the message body between 8 and 7 bits. If the SendMimeErrors option (SendMimeErrors) is set, V8 sendmail includes the following header in all returned (bounced) mail:

MIME-Version: 1.0

This is hardcoded into sendmail. See the SendMimeErrors option for further details about this header.

The MIME-Version: header should never be declared in the configuration file.