sendmail, 4th Edition

If the mail message contained a Precedence : header (see Section 25.10), the class= reflects sendmail 's interpretation of the keyword that follows that header. For example, given the configuration command:

Plist=-30

the following header will yield a class= value of -30 :

Precedence: list

If no Precedence : header is present in the message, the value shown for class= is zero. The class= is shown only for sender records.