HP-UX Mailing Services Administrator's Guide

Configuring and Administering Sendmail
Configuring Sendmail to Reject Unsolicited Mail
Chapter 290
Spam Control Using the Message Submission Agent
(RFC 2476)
Sendmail supports RFC 2476, a protocol for message submission. The
anti-spam rulesets have been enhanced to improve the anti-spam
capabilities. The RFC proposes a new standard for the Message
Submission Agent (MSA). This is designed to replace the more
general-purpose Mail Transfer Agent (MTA) as the first service to which
a Mail User Agent (MUA) connects to deliver a mail message. The RFC
also describes how the usual protocols for SMTP service must be
tightened up at the point where mail enters the system, rather than
being routed from one site to another. Sendmail also serves as a powerful
tool to authenticate and control mail messages.
By default, MSA is defined in the sendmail.cf file as:
O DaemonPortOptions=Name=MSA, Port=587, M=E
where Port 587 is reserved for e-mail message submission.
An MSA still uses the same rulesets for processing the message (and
therefore still allows message rejection via the check rulesets). In
accordance with the RFC, the MSA ensures that all domains in the
envelope are fully qualified if the message is relayed to another MTA. It
also enforces the normal address syntax rules and log error messages. In
addition, you can request authentication before the messages are
accepted by MSA by using the M=a modifier in the DaemonPortOptions.
NOTE You can turn off MSA in the sendmail.cf file using the option,
no_default_msa in the gen_cf script. For more information, see the
no_default_msa option in “Modifying the Default Sendmail
Configuration File” on page 48.
The XUSR SMTP command and the -U (initial user submission)
command-line option are deprecated. Mail user agents must use the
MSA (Message Submission Agent) for initial user message submission.
XUSR may be removed in future releases. The next release of Sendmail
will assume that any message submitted from the command line is an
initial user submission and act accordingly.