HP-UX Mailing Services Administrator's Guide

Configuring and Administering Sendmail
Configuring Sendmail to Reject Unsolicited Mail
Chapter 286
Relay Hosts Only: Relaying from Hosts Only
By default, host names that are listed as RELAY in both the Access
Database and the class R ($=R) macro can relay messages. When using
this feature, specify host names. This feature enables Sendmail to look
up individual host names and relay messages to the host.
See “Checking Headers” on page 88 for information on using the R class.
Relaying Based on MX Records
This feature allows relaying based on the MX records of the host portion
of an incoming recipient. If an MX record for host foo.com points to your
site, you will accept and relay mail addressed to foo.com.
Relay from Local
With this feature, a sender who is a valid user on a particular host can
relay messages to other users on different hosts.
IMPORTANT Use caution when using this feature. Using this feature opens a window
for spammers. Specifically, spammers can send mail to your mail server
that claim to be from your domain (either directly or via a routed
address), and your machine will relay it out to any hosts on the Internet.
Check Loose Relay
This feature turns off the default behavior, which rechecks all recipients
using % addressing. For example, if the recipient address is
user%site@othersite, and
othersite is in class R macro, Sendmail
strips the @othersite portion and rechecks user@site for relaying.
Validating Senders
Sendmail provides a stringent check of mail message senders to ensure
that they are legitimate. Sendmail refuses mail if the MAIL FROM:
parameter has an unresolvable domain. You can work around this. If you
want to continue accepting mail from such domains, use the features
described in this section. You can enable any of the following features
when you run the gen_cf script:
• Accept Unresolvable Domains