HP-UX Mailing Services Administrator's Guide

Mailing Services Overview
The Sendmail Utility
Chapter 132
otherwise inaccessible. For information on creating MX records, see
HP-UX IP address and Client Management Guide at the URL
http://www.docs.hp.com/hpux/netcom/index.html#Internet%20Se
rvices.
MX records are used only if a message address resolves to an IPC mailer
(that is, one that uses SMTP over sockets to perform delivery). Instead of
attempting to connect directly to the recipient host, Sendmail first
queries the name server, if it is running, for MX records for that host. If
the name server returns any answer, Sendmail sorts them in preference
order, highest preference (lowest number) first. If the local host appears
in the list, the local host and any MX hosts with lower preference (higher
numbers) are removed from the list. If any MX hosts remain, Sendmail
then tries to connect to each MX host in the list in order, and it delivers
the message to the first MX host to which it successfully connects. If that
MX host is not the final destination for the message, it is expected that the
host will relay the message to its final destination.
If Sendmail tries all the MX hosts in the list and fails, the message is
returned to the sender with an error message. If you want Sendmail to
try to connect to the host to which the message is addressed, uncomment
the following line in the /etc/mail/sendmail.cf file:
TryNullMXList
Sendmail then tries to connect to the host to which the message is
addressed, if any of the following conditions occur:
• The name server returns no MX records.
• The name server is not running.
• The local host is the highest preference mail exchanger in the list.
At log level 11 and above, Sendmail logs in the system log the name and
Internet address of the MX host (if any) to which it delivered (or
attempted to deliver) a message.
MX records are used for two main purposes:
• To arrange one host backup by receiving mail for the host when it is
down
• To arrange the mail addressed to remote networks be relayed
through the appropriate gateways
In the following example, the name server serving the domain paf.edu
has the following MX records configured to provide backup for host bling: