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

For more technical and conceptual information about Sendmail, HP recommends that
you read Bryan Costales and Eric Allman, 2001. Sendmail, 3rd Edition, O'Reilly and
Associates, Inc. You can also refer to the Sendmail 8.13 Companion by Bryan Costales.
For information about using Sendmail with BIND, HP recommends that you read DNS
and BIND, by Paul Albitz and Cricket Liu, also published by O’Reilly and Associates,
Inc.
You can get information about the O’Reilly books (availability, how to order them, and
so on) by visiting the O’Reilly website:
http://www.oreily.com
You also can visit the website for Sendmail:
http://www.sendmail.org
NOTE: All referrences of the term Sendmail in this document refer to Sendmail 8.13.3.
This section discusses the following topics:
• “Message Structure” (page 23)
• “How Sendmail Collects Messages” (page 24)
• “How Sendmail Routes Messages” (page 24)
• “Defining Queue Groups” (page 29)
• “How Sendmail Improves Mail Queue Performance” (page 32)
• “Default Client/Server Operation” (page 33)
• “How Sendmail Handles Errors” (page 34)
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.
The Sendmail Utility 23