Installing and Administering Internet Services

Chapter 4 135
Installing and Administering sendmail
How sendmail Works
How sendmail Works
sendmail acts as a post office to which all messages can be submitted
for routing. sendmail can interpret both Internet-style addressing (that
is, user@domain) and UUCP-style addressing (that is, host!user). How
addresses are interpreted is controlled by the sendmail configuration
file. sendmail can rewrite message addresses to conform to standards
on many common target networks.
This section discusses the following topics:
• “Message Structure” on page 135
• “How sendmail Collects Messages” on page 136
• “How sendmail Routes Messages” on page 136
• “Default Client-Server Operation” on page 142
• “How sendmail Handles Errors” on page 144
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 the programs that create, route, and
deliver the message. It is usually not seen directly by either the sender or
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.