A DNS-based Blackhole List (DNSBL) or Real-time Blackhole List (RBL) is an effort to stop email spamming. It is a “blacklist” of locations on the Internet reputed to send email spam. The locations consist of IP addresses which are most often used to publish the addresses of computers or networks linked to spamming; most mail server software can be configured to reject or flag messages which have been sent from a site listed on one or more such lists. The term “Blackhole List” is sometimes interchanged with the term “blacklist” and “blocklist”.
A DNSBL is a software mechanism, rather than a specific list or policy. There are dozens of DNSBLs in existence, which use a wide array of criteria for listing and delisting of addresses. These may include listing the addresses of zombie computers or other machines being used to send spam, ISPs who willingly host spammers, or those which have sent spam to a honeypot system.
Since the creation of the first DNSBL in 1997, the operation and policies of these lists have been frequently controversial, both in Internet advocacy and occasionally in lawsuits. Many email systems operators and users consider DNSBLs a valuable tool to share information about sources of spam, but others including some prominent Internet activists have objected to them as a form of censorship. In addition, a small number of DNSBL operators have been the target of lawsuits filed by spammers seeking to have the lists shut down.[Wiki]
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In case you haven’t used or heard of Zimbra,
Zimbra is an enterprise-class email, calendar and collaboration solution built for the cloud, both public and private. With a redesigned browser-based interface, Zimbra offers the most innovative messaging experience available today, connecting end users to the information and activity in their personal clouds.
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Add RBL check on Zimbra
Login to email server and su youreself to zimbra user.
# su - zimbra
Check current settings
$ zmprov gacf | grep zimbraMtaRestriction
zimbraMtaRestriction: reject_non_fqdn_sender zimbraMtaRestriction: reject_non_fqdn_sender
reject_non_fqdn_sender is set.
Add a test RBL server
$ zmprov mcf \ zimbraMtaRestriction reject_invalid_helo_hostname \ zimbraMtaRestriction reject_non_fqdn_sender \ zimbraMtaRestriction "reject_rbl_client cbl.abuseat.org"
I used \ to break the lines. You can do it all in one line if you feel like.
$ zmprov mcf zimbraMtaRestriction reject_invalid_helo_hostname zimbraMtaRestriction reject_non_fqdn_sender zimbraMtaRestriction "reject_rbl_client cbl.abuseat.org"
$ zmprov gacf | grep zimbraMtaRestriction zimbraMtaRestriction: reject_non_fqdn_sender zimbraMtaRestriction: reject_invalid_helo_hostname zimbraMtaRestriction: reject_rbl_client cbl.abuseat.org
Adding multiple RBL servers in Zimbra
Going full on retard with RBL check
$ zmprov mcf \ zimbraMtaRestriction reject_invalid_helo_hostname \ zimbraMtaRestriction reject_non_fqdn_sender \ zimbraMtaRestriction reject_invalid_hostname \ zimbraMtaRestriction "reject_rbl_client sbl.spamhaus.org" \ zimbraMtaRestriction "reject_rbl_client bl.spamcop.net" \ zimbraMtaRestriction "reject_rbl_client dnsbl.sorbs.net" \ zimbraMtaRestriction "reject_rbl_client cbl.abuseat.org" \ zimbraMtaRestriction "reject_rbl_client dnsbl.njabl.org"
$ zmprov gacf | grep zimbraMtaRestriction zimbraMtaRestriction: reject_invalid_helo_hostname zimbraMtaRestriction: reject_non_fqdn_sender zimbraMtaRestriction: reject_invalid_hostname zimbraMtaRestriction: reject_rbl_client sbl.spamhaus.org zimbraMtaRestriction: reject_rbl_client bl.spamcop.net zimbraMtaRestriction: reject_rbl_client dnsbl.sorbs.net zimbraMtaRestriction: reject_rbl_client cbl.abuseat.org zimbraMtaRestriction: reject_rbl_client dnsbl.njabl.org
List of RBL servers:
Don’t go full retard with RBL in Zimbra; quite often some RBL servers blacklist good domains for absolutely no reasons (unexplained); so test; test; test until you have the best combination. Here’s a list of all the RBLs/DNSBls you can check your mail servers against(mostly free):