HP-UX Mailing Services Administrator's Guide

Mailing Services Overview
The Sendmail Utility
Chapter 128
3. Passes the mail to one of the several specialized delivery agents for
delivery.
Figure 1-1 outlines the flow of messages through Sendmail.
After Sendmail collects a message, it routes the message to each of the
specified recipient addresses. In order to route a message to a particular
address, Sendmail must resolve that address to a {
delivery agent ,
host , user } triple. This resolution is based on the rules defined in the
Sendmail configuration file, /etc/mail/sendmail.cf.
Sendmail invokes a separate delivery agent for each host to which
messages are being routed. Some delivery agents can accept multiple
users in a given invocation. Others must be invoked separately for each
recipient. Delivery agents that HP supports for use with Sendmail
include SMTP, UUCP, X.400, and OpenMail.
To invoke a delivery agent, Sendmail constructs a command line
according to a template in the configuration file. If the delivery agent is
specified as IPC, Sendmail does not invoke an external delivery agent.
Instead, Sendmail opens a TCP/IP connection to the SMTP server on the
specified host and transmits the message using SMTP.