2.2.3.2.2 Content-Type Header Field
The Content-Type header field specifies the type of content. The Content-Type header MUST contain the string "application/mapi-http" on requests. The Content-Type header MUST contain the string "application/mapi-http" on responses with X-ResponseCode header of 0. If X-ResponseCode is non-zero, the Content-Type header MUST contain the string "text/html".