{{Unreferenced, date=March 2009 Context filtering is an

anti-spam Various anti-spam techniques are used to prevent email spam (unsolicited bulk email). No technique is a complete solution to the spam problem, and each has trade-offs between incorrectly rejecting legitimate email (false positives) as opposed to ...

/ mail policy method that does not deal with the contents of the mail but rather uses the context of the

SMTP The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typic ...

connection to decide whether a mail will be accepted or not. This method usually prevents reception of an e-mail in the first place; thus, common anti-spam features like quarantine, redirect or delete can not be applied. This method also distorts statistics of anti-spam programs because it is usually unknown how many mails (Spam or otherwise) would have been received through a certain connection. Filtering by context includes methods like

DNSBL A Domain Name System blocklist, Domain Name System-based blackhole list, Domain Name System blacklist (DNSBL) or real-time blackhole list (RBL) is a service for operation of mail servers to perform a check via a Domain Name System (DNS) query whe ...

lookups, domain or IP blacklists, etc. This method has an extremely low rate of

false positive A false positive is an error in binary classification in which a test result incorrectly indicates the presence of a condition (such as a disease when the disease is not present), while a false negative is the opposite error, where the test resul ...

s and

false negative A false positive is an error in binary classification in which a test result incorrectly indicates the presence of a condition (such as a disease when the disease is not present), while a false negative is the opposite error, where the test result ...

s. On the plus side, this method is usually saving

bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

and server utilization. The most common context filter is DNSBL filtering. The efficiency of this method depends on the blacklist used. For high volume mail sites, it is recommended to set up local copies of the DNSBLs of your choice and fill them through

DNS The Domain Name System (DNS) is a hierarchical and distributed naming system for computers, services, and other resources in the Internet or other Internet Protocol (IP) networks. It associates various information with domain names assigned to ...

zone transfer DNS zone transfer, also sometimes known by the inducing DNS query type AXFR, is a type of DNS transaction. It is one of the many mechanisms available for administrators to replicate DNS databases across a set of DNS servers. A zone transfer uses ...

s. This reduces the permanent traffic through external DNS lookups. Some DNSBLs also allow only a certain amount of queries by day for every

IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...

to keep their expenses on bandwidth low.

See also