The address must be
composed of elements that create an identifier that is truly unique worldwide. The Message-
ID: header is declared in the configuration file:
H?M?Message-Id: <$t.$i@$j>
Here, the field is an address of the form identifier@domain, which is enclosed in angle
brackets. The $t macro (?§21.9.92 on page 846) is an integer representation of the current
time to the nearest second. The $i macro (?§21.9.52 on page 826) is the unique queue identifier
that is used to identify this message locally. The $j macro (?§21.9.59 on page 830) is
the fully qualified domain name of the local host. The Message-ID: header as it might
appear in an actual mail message would look like this:
Message-Id: <200210141542.g9EFg2bb006638@nic.cerf.net>
The Message-ID: header should be prefixed with a ?M? flag so that it is inserted only into
headers of messages whose delivery agents have the F=M flag set. The standard delivery
agents include this flag.
The resent- form of the Message-ID: header must also be declared in the configuration file:
H?M?Resent-Message-Id: <$t.$i@$j>
This ensures that every mail message has a message identifier even if the message is
forwarded.
Note that sendmail does not add a Message-ID: header or its Resent- form if a Message-ID:
header already exists in the header portion of a mail message. Furthermore, the Resentform
is added only if sendmail determines that the message is a re-sent message.
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