Therefore, this document defines a new header field, "MIME- Version", which is to be used to declare the version of the Internet message body format standard in use.
Messages composed in accordance with this document MUST include such a header field, with the following verbatim text:
MIME-Version: 1.0The presence of this header field is an assertion that the message has been composed in compliance with this document.
Since it is possible that a future document might extend the message format standard again, a formal BNF is given for the content of the MIME-Version field:
MIME-Version := textThus, future format specifiers, which might replace or extend "1.0", are (minimally) constrained by the definition of "text", which appears in RFC 822.
Note that the MIME-Version header field is required at the top level of a message. It is not required for each body part of a multipart entity. It is required for the embedded headers of a body of type "message" if and only if the embedded message is itself claimed to be MIME-compliant.