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.