Understanding Microsoft Message-ID Email Header

Alfi D 6 Reputation points
2021-09-06T07:09:14.4+00:00

Hi All,

I would like to understand further on how Microsoft generates its message-id that is found in the email headers.

I noticed the message-id ended with "prod.outlook.com" at the end of the message-id

<yyyyyyyyyyyyyxxxxxxxxxxxxxxxxxxxxxxxxx@yyyyyyyyyyyyy.zzzzzzzz.prod.outlook.com>

I would like to know what the different parts of the message-id signifies and if it tells us about the location of the server the email went through.

Thanks in advance!

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  1. Andy David - MVP 149.5K Reputation points MVP
    2021-09-06T15:10:20.12+00:00

    The Message-ID contains the name of the server that generated it, yes:
    the part after the @
    Example: "BLAPR05MB7491.namprd05.prod.outlook.com"
    The location is the part before outlook.com
    So in this example: namprod05 = North America.
    The message hops in the internet header will tell you all the servers the message routed through.

    Per the RFC:

    https://datatracker.ietf.org/doc/html/rfc2822

    The "Message-ID:" field provides a unique message identifier that
    refers to a particular version of a particular message. The
    uniqueness of the message identifier is guaranteed by the host that
    generates it (see below). This message identifier is intended to be
    machine readable and not necessarily meaningful to humans. A message
    identifier pertains to exactly one instantiation of a particular
    message; subsequent revisions to the message each receive new message
    identifiers.

    Note: There are many instances when messages are "changed", but those
    changes do not constitute a new instantiation of that message, and
    therefore the message would not get a new message identifier. For
    example, when messages are introduced into the transport system, they
    are often prepended with additional header fields such as trace
    fields (described in section 3.6.7) and resent fields (described in
    section 3.6.6). The addition of such header fields does not change
    the identity of the message and therefore the original "Message-ID:"
    field is retained. In all cases, it is the meaning that the sender
    of the message wishes to convey (i.e., whether this is the same
    message or a different message) that determines whether or not the
    "Message-ID:" field changes, not any particular syntactic difference
    that appears (or does not appear) in the message.

    3 people found this answer helpful.

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