Anti-spam software is designed to filter and prevent unwanted and junk email (spam). Like other types of filtering software, anti-spam products apply user-defined criteria or pre-set filters to exclude, block, quarantine, or prevent the delivery of unsolicited e-mail to a user’s inbox. Email monitoring and spam filtering software may filter incoming email by sender, recipient, subject, or body. Spam prevention software that filters by sender is designed to exclude known spam domains. Spam blocking software that filters by recipient enables users to reject e-mails that do not contain their own e-mail address in the recipient field. Anti-spam software that filters by subject enables users to exclude email that contains specific words or phrases in the subject line. Similarly, spam filtering software that monitors incoming email by body blocks messages that contain specific header or body text. Advanced spam blockers such as Baynesian filters are also available. Anti-spam software uses various types of spam filtering techniques. Products that require actions by individuals are designed for non-administrative computer users who send and receive email. Typically, these spam blockers are rule-based. Spam filtering software for e-mail administrators is also commonly available. Typically, these anti-spam applications are challenge/response (C/R) systems with features such as checksum-based filtering and even country-based filtering. Anti-spam software for e-mail administrators may also feature DNS-based blacklisting, greeting delays, invalid pipelining, no-listing, quit detection, honeypots, hybrid filtering, statistical content filtering, tarpits, and other spam trapping (spamtrapping) techniques. Some e-mail senders use anti-spam software to ensure that their bulk mailings do not cause their IP addresses to be placed on a DNS-based black hole list (DNSBL), block list, or blacklist. Automated anti-spamming techniques for bulk e-mail senders include user background checks, confirmed opt-in for mailing lists, egress filtering, backscatters, port 25 blocking, and port 25 interception. Features for such anti-spam software may also include rate limiting, spam report feedback loops, and well-defined acceptable use policy (AUP) and terms of service (TOS) agreements. Specialized anti-spam software is designed to filter and prevent instant messaging spam (IM spam), newsgroup and forum spam, mobile phone spam, online game messaging spam, search engine spam, blog spam, and video spam.