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 internal variables.

$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).