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DNSBL Usage Terms
How Blocklists Work
ZEN is the combination of all Spamhaus IP-based DNSBLs into one single powerful and comprehensive blocklist to make querying faster and simpler. It contains the SBL, SBLCSS, XBL and PBL blocklists.
zen.spamhaus.org should be the only spamhaus.org DNSBL in your IP blocklist configuration. You should not use ZEN together with other Spamhaus IP blocklists, or with blocklists already included in our zones (such as the CBL) or you will simply be wasting DNS queries and slowing your mail queue.
zen.spamhaus.org replaces sbl-xbl.spamhaus.org in most configurations. If you are currently using sbl-xbl.spamhaus.org you should replace sbl-xbl.spamhaus.org with zen.spamhaus.org.
Caution: Because ZEN includes the XBL and PBL lists, do not use ZEN on smarthosts or SMTP AUTH outbound servers for your own customers (or you risk blocking your own customers). Do not use ZEN in filters that do any ‘deep parsing’ of Received headers, or for anything other than checking IP addresses that hand off to your mailservers.
ZEN Return Codes
Querying zen.spamhaus.org returns the following result codes for listed entries (see the FAQs for a more complete return code table):
The ZEN blocklist can be used by all modern mail servers, by setting your mail server's anti-spam DNSBL feature (sometimes called "Blocklist Servers" or "RBL servers") to query: zen.spamhaus.org
Before including this blocklist as part of your mail server's spam filters please see Spamhaus's DNSBL Usage Terms to check if you qualify for free use.