HP-UX Mailing Services Administrator's Guide (B2355-91064)

How Sendmail Collects Messages
Sendmail receives messages through any of the following methods:
• A user agent calls Sendmail to route a piece of mail. User agents supported by HP
for use with Sendmail are elm, mail, and mailx.
• A Sendmail daemon or other mail program calls Sendmail to route a piece of mail
received from the network or the mail queue.
• A user invokes Sendmail directly from the command line.
How Sendmail Routes Messages
Sendmail routes messages as follows:
1. Rewrites the recipient and sender addresses given to it, to comply with the
standards of the target network.
2. If necessary, adds lines to the message header to enable the recipient to reply.
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.
24 Mailing Services Overview