When sendmail bounces a mail message, it needs to create headers that probably didn't exist before. It uses the putheader( ) routine to create them. The -d34.1 (a.k.a. -d34) debugging switch causes sendmail to print the following on entry to that routine:

--- putheader, mailer = agent —-

Here, agent is the symbolic name of the delivery agent that will deliver the bounced message.