The $c macro is used to
store the number of times a mail message has been forwarded from site
to site. It is a count of the number of Received:,
Via:, and Mail-From: header
lines in a message.
The value in $c is not used by
sendmail. Rather, it is made available for use
in configuration-file header-line definitions. When calculating the
hop count for comparison to the MaxHopCount option
(MaxHopCount), sendmail uses
$c is transient. If 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 $&c, not $c).