If a Message Processing Failure returns a 'Bad-data' response with the following description 'The Content-Type Header is Invalid', the gateway cannot recognise the message header.
Refer to http://www.ietf.org/rfc/rfc2045.txt and see section 5.1 - Syntax of the Content-Type Header Field for further information.

In the Augmented BNF notation of RFC 822, a Content-Type header field value is defined as follows:

content := "Content-Type" ":" type "/" subtype
*(";" parameter)
; Matching of media type and subtype
; is ALWAYS case-insensitive.

type := discrete-type / composite-type

discrete-type := "text" / "image" / "audio" / "video" /
"application" / extension-token

composite-type := "message" / "multipart" / extension-token

In this case, the original mailer has not composed the mail correctly as they have missed out the discrete type.
Currently the message is not RFC compliant.

It is recommended that the sending host of the message should be contacted and informed of this error so they can resolve the malformed mime content.