Spam filter
Also called: anti-spam filter
A spam filter is a computer program that analyzes e-mail messages and determines whether they are spam. Spam filters are used to reduce the nuisance of unwanted e-mail. When the filter detects a spam message, action is taken. Depending on the program and how it is set up, the message may be deleted, moved to a specific folder, or a warning may be added to the subject.
A number of techniques exist for assessing whether an e-mail message is spam.
A commonly used technique is to assess the characteristics and content of the e-mail message. Suspicious characteristics are assigned points. Once the total number of points exceeds a set value, the message is marked as spam.
Examples of features that may indicate spam:
- Suspicious words in the text (such as drug names);
- Use of certain HTML tags;
- Potentially hazardous attachments;
- Large amounts of identical emails.
Another method looks at whether the sender's e-mail address or IP address has previously been used to send unsolicited e-mail. Some blacklists exist on the Internet that track (potential) senders of spam. These lists are often maintained automatically by affiliated spam filters, which use their findings on the e-mail traffic they see passing by.
Spam filters can be installed either on a server that handles e-mail or on the recipient's computer. Many Internet service providers scan incoming e-mail for spam by default. Many e-mail clients (such as Outlook) also have a built-in spam filter.
A regular e-mail message, such as an e-mail newsletter, can also be unfairly classified as spam. For example, because the IP address of the web server used to send the message is shared by multiple websites, and one of these websites has sent spam. Or because the e-mail message itself meets sufficient characteristics to be considered spam. Especially with nicely formatted e-mailings, it is not always easy to make these messages pass any spam filter.
E-mail recipients often have the ability to exempt (whitelist) a particular e-mail address within the spam filter, so that all e-mail coming from that address gets through anyway.
