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Glossary - Blacklists

RBL Check
"Blacklists," preferably known by some as "block lists," are usually databases of IP addresses that are known to send spam.  These databases can be utilized by most eMail server software to cross-check the source of all eMail messages in real-time.  If the cross-check yields a positive response (because the sender is blacklisted), the connection can be cut off automatically before message delivery can succeed.

If you find that an eMail message you sent gets rejected due to blacklisting, you could visit the referenced web page for a more detailed explanation (a link will usually be included), but you'll probably just want to contact your ISP to ask them to get de-listed so that you can send eMail again.  Note that contacting blacklist operators to request a de-listing normally only works for your ISP's staff who operate the eMail system.


Spammers often view this concept of blocking based on a DNSBL as a form of unfair censorship by the blacklist operators, but in reality the recipients generally agree with the blacklist operator's criteria and have chosen to make use of it to stop spam from getting into their systems.

Whomever has the authority over the eMail servers decides on blocking policies (usually the mail server administrator, with support from the owner{s} or upper management if the servers aren't their own).  The use of a particular DNSBL for the purpose of blocking incoming connections is justified for many reasons, a few of which may include a cost-savings by relying on the specialized expertise of the DNSBL's operators to save time (because the administrator doesn't have to maintain their own DNSBL), agreement with the DNSBL's criteria (and how well this criteria is managed), etc.

In the context of blocking, the use of a DNSBL is usually to determine if a particular IP address (or internet domain name) is blacklisted.  Consequently, the configuration of the mail server to block based on this information is the decision of the mail server administrator -- ultimately it is the mail server administrator who has made the decision to block, albeit indirectly through delegation.

Blaming a DNSBL is like shooting a messenger for delivering bad news.

More information

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