The sendmail program calls other programs (called mail delivery agents) to perform that delivery. Some delivery agents are external (as for local delivery), and others are internal (as for network delivery). The mailer= syslog equate shows the symbolic name (Section 20.1) of the delivery agent that was used to perform delivery to the recipient:

mailer=agent

A list of symbolic names assigned to delivery agents can be viewed with the -d0.15 debugging switch (-d0.15). The mailer= syslog equate is shown only for recipient records.