2.1.3.2.11 Message ID

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MIME writers SHOULD copy the value of the PidTagInternetMessageId property ([MS-OXOMSG] section 2.2.1.13) to the Message-ID header. A MIME writer MAY<38> generate a new Message-ID header if it is generating a message to send via SMTP. If no value is specified for the PidTagInternetMessageId property when a message is submitted to SMTP, MIME writers SHOULD generate a value as specified in [RFC2822].

Clients SHOULD NOT set the PidTagInternetMessageId property when submitting a message via remote procedure call (RPC). As specified in [RFC2822], the value of the Message-ID header is unique and is assigned by the server that originated the message. Servers MAY overwrite the PidTagInternetMessageId property from a client before submitting the message to SMTP.

Once set, the value of the Message-ID header and the corresponding value of the PidTagInternetMessageId property SHOULD remain constant. MIME writers SHOULD NOT overwrite the value of the PidTagInternetMessageId property when generating MIME messages for protocols such as POP3 or IMAP4.