Administering Mail and Messaging

Blocking spam email using the MAPS Realtime Blackhole List

The check_relay ruleset in the configuration file for UnixWare 7 sendmail(1M) can be configured to reject mail based upon the MAPS (Mail Abuse Prevention System) Realtime Blackhole List maintained at The MAPS RBL is a list of sites which are known to either originate or relay spam. During an SMTP session, check_relay makes a DNS query on the IP address of the connecting host to check for the existence of an address resource record (``A'') of that host under the node. If such a record exists, then the check_relay ruleset issues an error and drops the SMTP connection.

The check_relay ruleset follows the following algorithm:

If the mail is rejected, sendmail issues the following error message during the SMTP dialogue, and writes it to the system log file as well:
   Mail from <connecting host> refused by blackhole site

By default, this feature is disabled in To enable check_relay, set the Check_Relay macro to a non-null value by changing the following line in


to the following:

   D{Check_Relay} ON

Next, stop and restart the sendmail daemon to load the configuration changes by executing the following commands:

/etc/mail/sendmailrc stop
/etc/mail/sendmailrc start

NOTE: Use of the check_relay ruleset requires that DNS be properly configured, and is incompatible with the DeliveryMode=deferred sendmail option, as it requires a DNS lookup be done immediately upon mail receipt. Administrators should also consider whether their server can handle the load of an additional DNS lookup for each message received before enabling this feature.

© 1999 The Santa Cruz Operation, Inc. All rights reserved.
UnixWare 7 Release 7.1.1 - 5 November 1999