HP-UX Mailing Services Administrator's Guide

Mailing Services Overview
The Sendmail Utility
Chapter 1 27
• “Default Client/Server Operation” on page 35
• “How Sendmail Handles Errors” on page 36
Message Structure
A message has three parts: an envelope, a message header, and a
message body.
The envelope consists of the sender address, recipient address, and
routing information shared by programs that create, route, and deliver
the message. It is usually not seen directly by either the sender or the
recipients of the message.
The message header consists of a series of standard text lines used to
incorporate address, routing, date, and other information into the
message. Header lines may be part of the original message and may also
be added or modified by the various mail programs that process the
message. Header lines may or may not be used by these programs as
envelope information.
By default, the first blank line in the message terminates the message
header. Everything that follows is the message body and is passed
uninterpreted from the sender to the recipient.
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.