SpamAssassin Rule: MIME_HEADER_CTYPE_ONLY
Standard description: 'Content-Type' found without required MIME headers
The specified Content-type for the mail is something other than "text/plain", so the headers must conform to the MIME specification, but one or more of the following headers have not been provided:
This suggests that the message was generated by a badly-written mailout program rather than by a normal email client.
The default scores for this rule can be found in the online list of tests.
While the MIME-Version: header is mandatory (RFC 2045 § 4) the Content-Transfer-Encoding: header defaults to "7bit" if not present (RFC 2045 § 6.1) and the Content-Disposition: header is considered optional (RFC 2183 § 2).