Each queued message
is identified by a unique identifier (Section 11.2.1).
The $i macro contains as its value that
identifier. Prior to V8.6 sendmail,
$i had a value assigned to it only when a file was
first placed into the queue. Beginning with V8.6
sendmail, $i is also given a
value when the queue file is processed.
$i is not used by sendmail
internally, nor should you use it in a rule set. It should be trusted
for use only in the Received: and