A Content-Type of "message/rfc822" indicates that the body contains an encapsulated message, with the syntax of an RFC 822 message. However, unlike top-level RFC 822 messages, it is not required that each message/rfc822 body must include a "From", "Subject", and at least one destination header.
It should be noted that, despite the use of the numbers "822", a message/rfc822 entity can include enhanced information as defined in this document. In other words, a message/rfc822 message may be a MIME message.