Blocking spam email using the MAPS Realtime Blackhole List

The check_relay ruleset in the sendmail.cf 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 maps.vix.com. 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 rbl.maps.vix.com 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 name of the host making the SMTP connection is the local host, the local domain, or a sub-domain of the local domain, then the mail is accepted.

• If the IP address of the host making the SMTP connection is not listed at the MAPS RBL, then the mail is accepted.

• Otherwise, the mail is rejected.

   Mail from <connecting host> refused by blackhole site rbl.maps.vix.com

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

   D{Check_Relay}

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