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^[7] can be used to display the workspace being passed to each such rule set:

^[7] There is no -d48.1 information.

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.