When sendmail processes one of the check_ rule sets (Section 7.1), and when the rule set rejects a message, sendmail logs one of the following two messages:

... ruleset=rset, arg1=firstarg, discard 
... ruleset=rset, arg1=firstarg, reject=reason

Here, rset is the name of the rule set called (such as check_mail, Section 7.1.2). The workspace passed to the rule set is indicated by firstarg. When a message is rejected, a reason is passed back to the original envelope sender.