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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: 1.0
The 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:
version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
Thus, future format specifiers, which might replace or extend "1.0", are constrained to be two integer fields, separated by a period. If a message is received with a MIME-version value other than "1.0", it cannot be assumed to conform with this specification.
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-conformant.
It is not possible to fully specify how a mail reader that conforms with MIME as defined in this document should treat a message that might arrive in the future with some value of MIME-Version other than "1.0". However, conformant software is encouraged to check the version number and at least warn the user if an unrecognized MIME- version is encountered.
It is also worth noting that version control for specific content- types is not accomplished using the MIME-Version mechanism. In particular, some formats (such as application/postscript) have version numbering conventions that are internal to the document format. Where such conventions exist, MIME does nothing to supersede them. Where no such conventions exist, a MIME type might use a "version" parameter in the content-type field if necessary.
MIME-Version: 1.0 MIME-Version: 1.0 (Generated by GBD-killer 3.7)
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