The Message-ID: header is used to uniquely identify each mail message. This header must be declared in the configuration file. The field for this header must be an expression in the syntax of a legal address enclosed in angle brackets (< and >). 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:

?M?Message-Id: <$t.$i@$j>

Here, the field is an address of the form identifier@domain, which is enclosed in angle brackets. The $t macro ($t) is an integer representation of the current time to the nearest second. The $i macro ($i) is the unique queue identifier that is used to identify this message locally. The $j macro ($j) 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:

?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 Resent- form is added only if sendmail determines that the message is a re-sent message.

Also note that you should never try to replace an existing Message-ID: header with one of your own. This could result in the loss of important information needed to trace the origin of a message.