Address harvester
A program that searches web pages and filters newsgroup postings looking for valid email addresses to be used for spam purposes. (See also harvesting.)

Bayesian filtering
A statistical approach to determining whether an email is spam. Based on probability inference techniques pioneered by English mathematician Thomas Bayes. Bayesian filtering is one of the techniques used by Barracuda to determine if an email is spam or not.

Blackhole list
A publicized list, usually commercial, of IP addresses known to be sources of spam. which can be used to create a network blacklist to filter out mail originating from these addresses. (See also real-time blackhole list.)

Blacklist
A feature of anti-spam software that allows users to designate IP addresses, domain names and individual email addresses from which no mail will be accepted. (See also Domain Name System blackhole list.)

Complex dictionary checking
A feature of anti-spam software that screens text for rude words and isn't fooled by various spam tricks, such as the replacement of letters with look-alike numerals or characters (such as "1nterest r@te").

Denial of Service (DoS) attack
Where a hacker sends attachments or other unusual or excessive traffic in an attempt to bring down email systems.

Dictionary attack
A program that bombards a mail server with millions of alphabetically generated email addresses in the hope that some addresses will be guessed correctly. This technique is also used to crack passwords.

Directory Harvest Attack (DHA)
When a spammer bombards a domain with thousands of generated email addresses in an attempt to collect valid email addresses from an organization. (See also harvesting.)

Domain Name System blackhole list (DNSBL)
Commercial lists of networks that either allow spammers to use their systems to send spam, or have not taken action to prevent spammers from abusing their systems.

False negative
When anti-spam software fails to identify a spam message as spam.

False positive
When anti-spam software wrongly identifies a legitimate message as spam.

Greylist
Senders who are not blacklisted (excluded) or whitelisted (accepted) can be placed on a greylist. Some anti-spam software can send greylisted addresses an automated response, challenging the sender to confirm their legitimacy.

Hacker
Someone who intentionally breaches computer security, usually to cause disruption or gain confidential information such as financial details. Originally the word "hacker" referred to any person who was into computer technology, but is now commonly used by the public and media to refer to those who have malicious intentions.

Ham
All email that a recipient does not consider to be spam. (See also spam.)

Harvesting
The process of scanning the internet to identify email addresses in order to create lists for spamming.

NDR spam
Uses a faked standard email non-delivery report (NDR) that a recipient will think is genuine, tricking them into opening an attachment that is spam. Spammers can send such an NDR directly or make a legitimate server send it for them, adding to its credibility.

Network check (also known as reverse DNS check)
When an anti-spam engine uses a Domain Name System database to check an e-mail's IP address to ensure that it originated from a valid domain name or web address.

Newsgroup
An electronic forum where readers post articles and follow-up messages on specified topics. Often targeted by spammers seeking to harvest email addresses.

Obfuscation
Spammers' attempts to hide data to prevent its detection. Also, when email recipients use HTML or JavaScript to obscure mailto links and email addresses so that addresses remain readable and clickable, but cannot be harvested. (See also munging.)

Open relay
An SMTP email server that allows the third-party relay of email messages. The relay feature is a part of all SMTP-based servers and it has legitimate uses, but spammers have learned how to locate unprotected servers and hijack them to send spam.

Opt-in
The process of agreeing to receive email from a business source. Double opt-in refers to a double-check procedure in which a decision to be included on a mailing list is confirmed.

Opt-out
The process of declining to receive email from a business source or unsubscribing if the recipient is already on a mailing list.

Phishing
(Pronounced 'fishing'.) This involves creating a replica of a legitimate web page to hook users and trick them into submitting personal or financial information or passwords.

Real-time blackhole list (RBL)
This differs from a blackhole list in that it actively boycotts TCP/IP addresses known to send spam or host spammers. Enabling such a list results in all mail from those addresses being refused, including valid email. This can, however, result in innocent users complaining to their ISPs and those ISPs enacting stronger anti-spam measures in order to get the RBL ban lifted.

Social engineering
Conning email recipients into opening messages, revealing passwords or providing other confidential information by appealing to their curiosity, gullibility or computing naivety.

Spam
All unsolicited commercial email (UCE) and unsolicited bulk email (UBE) that a recipient does not want to receive. (See also CSS spam, NDR spam, and ham.)

Spoofing
When spammers forge an email address to hide the origin of a spam message. Email scammers and virus writers also use this trick. Scammers spoof address lines to fool people into thinking an email has arrived from a legitimate source, such as an online bank. Similarly, virus writers have passed off viruses as security patches by spoofing their origin as being, for example, from Microsoft technical support.

Web bug
A small graphic inserted in an email or web page that alerts a spammer when a message is read or previewed.

Whitelist
A list of external email addresses, IP addresses and domains trusted by the entire organization or individual users. All mail from these addresses is delivered, bypassing the spam filters.