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

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” (page 89) 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
• Accept Unqualified Senders
• Blacklist Recipients
• Realtime Blackhole List
Configuring Sendmail to Reject Unsolicited Mail 87