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A Domain Name Blacklist/Blocklist (DNSBL) is a list of domain names that have been blacklisted or blocked. There are many of DNSBLs available, all of which list and delist addresses based on a variety of criteria. Various DNSBLs are used by spam filters to ensure that incoming emails do not come from sites that have been blacklisted. DNSBLs are typically the initial line of spam defence.
SURBLs (no longer an acronym) are the second line of defence in most spam filters, but not all of them can use them. SURBLs are blacklists of websites that have been identified as being often used in unsolicited emails. Spam filters that support SURBLs will examine the body text of emails for any website addresses and compare them to the blacklists. If an address is discovered, the message will be marked as spam and treated as such.
Because the two approaches filter in distinct ways, they are most successful when used together. Regrettably, as spammers get more sophisticated, there is no method that can detect every spam letter. Given the methods used by spammers, it’s worth noting that DNSBLs change more frequently than SURBLs. If your server has been banned and you’re an email system administrator, you’ll need to work with the various DNSBLs and/or SURBLs that may have you listed (www.surbl.org).