Understanding Microsoft Message-ID Email Header

Question

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!

Answer 1

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.