Beginning with V8.8, sendmail calls rule sets
whose names begin with check_ (Section 7.1) to filter incoming and outgoing mail, to
accept or reject connections, and to decide on actions, such as
allowing STARTTSL. The -d48.2 debugging
switch can be used to display the workspace
being passed to each such rule set:
rscheck(ruleset, left, right)
The ruleset is the name of the named rule set
being called. If right is missing, it prints as
NULL, and the workspace passed to the rule set is:
left
If right is present, the workspace is:
left $| right
Here, the $| in the workspace is the
$| operator.