${msg_id}
Value of the Message-Id: header V8.13 and later
The Message-Id:
header (Message-ID: on page
1159) is used to uniquely identify each mail
message. It must be declared in the configuration
file. Its field must be an expression in the syntax
of a legal email address
(user@host) enclosed in
angle brackets (< and >) composed of elements that create
an identifier that is truly unique worldwide.
Beginning with V8.13, when sendmail finds a Message-Id: header in
the current message, it assigns the value for that
header to this ${msg_id} macro. If
sendmail finds no Message-Id: header, it
creates one and assigns that new value to this
${msg_id}
macro.
If a Message-Id:
header appeared in the original inbound message, its
value can be made available to rule sets by using
the H
configuration command (Rules Check Header Contents on page
1130) and to Milters using an xxfi_header() routine
(Milter xxfi_header() on page 1217). But if sendmail creates the Message-Id: header, its
value can be made available only by using this
${msg_id}
macro.
Be aware that ${msg_id} is transient. If it is
defined in the configuration file or in the command
line, that definition can be ignored by
sendmail. Note that a
$& prefix
is necessary when you reference this macro in rules
(that is, use $&{msg_id}, not ${msg_id}).
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