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Anti-spam techniques can be broken into four broad categories: those that require actions by individuals, those that can be automated by e-mail administrators, those that can be automated by e-mail senders and those employed by researchers and law enforcement officials.
People tend to be much less bothered by spam slipping through filters into their mail box (false negatives), than having desired e-mail ("ham") blocked (false positives). Trying to balance false negatives (missed spams) vs false positives (rejecting good e-mail) is critical for a successful anti-spam system. Some systems let individual users have some control over this balance by setting "spam score" limits, etc. Most techniques have both kinds of serious errors, to varying degrees. So, for example, anti-spam systems may use techniques that have a high false negative rate (miss a lot of spam), in order to reduce the number of false positives (rejecting good e-mail).
Detecting spam based on the content of the e-mail, either by detecting keywords such as "viagra" or by statistical means (content or non-content based), is very popular. Content based statistical means or detecting keywords can be very accurate when they are correctly tuned to the types of legitimate email that an individual gets, but they can also make mistakes such as detecting the keyword "cialis" in the word "specialist" (see also Internet censorship: Over- and under-blocking). The content also doesn't determine whether the email was either unsolicited or bulk, the two key features of spam. So, if a friend sends you a joke that mentions "viagra", content filters can easily mark it as being spam even though it is neither unsolicited nor sent in bulk. Non-content base statistical means can help lower false positives because it looks at statistical means vs. blocking based on content/keywords. Therefore, you will be able to receive the friend who sends you a joke that mentions "viagra".
The most popular DNSBLs (DNS Blacklists) are lists of IP addresses of known spammers, known open relays, known proxy servers, compromised “zombie” spammers, as well as hosts on the internet that shouldn’t be sending external emails, such as the end-user address space of a consumer ISP. These are known as “Dial Up Lists”, from the time when end users had to dial up to the internet with a modem and a phone line.
Spamtraps are often email addresses that were never valid or have been invalid for a long time that are used to collect spam. An effective spamtrap is not announced and is only found by dictionary attacks or by pulling addresses off hidden webpages. For a spamtrap to remain effective the address must never be given to anyone. Some black lists, such as spamcop, use spamtraps to catch spammers and blacklist them.
Enforcing technical requirements of the Simple Mail Transfer Protocol (SMTP) can be used to block mail coming from systems that are not compliant with the RFC standards. A lot of spammers use poorly written software or are unable to comply with the standards because they do not have legitimate control of the computer sending spam (zombie computer). So by setting restrictions on the mail transfer agent (MTA) a mail administrator can reduce spam significantly, such as by enforcing the correct fall back of Mail eXchange (MX) records in the Domain Name System, or the correct handling of delays (Teergrube).
There are a number of techniques that individuals can use to restrict the availability of their e-mail addresses, reducing or preventing their attractiveness to spam.
Sharing an email address only among a limited group of correspondents is one way to limit spam. This method relies on the discretion of all members of the group, as disclosing email addresses outside the group circumvents the trust relationship of the group. For this reason, forwarding messages to recipients who don't know one another should be avoided. When it is absolutely necessary to forward messages to recipients who don't know one another, it is good practice to list the recipient names all after "bcc:" instead of after "to:". This practice avoids the scenario where unscrupulous recipients might compile a list of email addresses for spamming purposes. This practice also reduces the risk of the address being distributed by computers affected with email address harvesting malware. However, once the privacy of the email address is lost by divulgence, it cannot likely be regained.
Posting anonymously, or with a fake name and address, is one way to avoid e-mail address harvesting, but users should ensure that the fake address is not valid. Users who want to receive legitimate email regarding their posts or Web sites can alter their addresses so humans can figure out but spammers cannot. For instance, joe@example.com might post as joeNOS@PAM.invalid.example.com. Address munging, however, can cause legitimate replies to be lost. If it's not the user's valid address, it has to be truly invalid, otherwise someone or some server will still get the spam for it. Other ways use transparent address munging to avoid this by allowing users to see the actual address but obfuscate it from automated email harvesters with methods such as displaying all or part of the e-mail address on a web page as an image, a text logo shrunken to normal size using in-line CSS, or as jumbled text with the order of characters restored using CSS.
Spammers often regard responses to their messages—even responses like "Don't spam me"—as confirmation that an email address is valid. Likewise, many spam messages contain Web links or addresses which the user is directed to follow to be removed from the spammer's mailing list. In several cases, spam-fighters have tested these links, confirming they do not lead to the recipient address's removal—if anything, they lead to more spam. This removal request of filing a complaint may get the address list washed. To lower complaints so the spammer can stay active before having to acquire new accounts and/or internet provider.
People tend to be much less bothered by spam slipping through filters into their mail box (false negatives), than having desired e-mail ("ham") blocked (false positives). Trying to balance false negatives (missed spams) vs false positives (rejecting good e-mail) is critical for a successful anti-spam system. Some systems let individual users have some control over this balance by setting "spam score" limits, etc. Most techniques have both kinds of serious errors, to varying degrees. So, for example, anti-spam systems may use techniques that have a high false negative rate (miss a lot of spam), in order to reduce the number of false positives (rejecting good e-mail).
Detecting spam based on the content of the e-mail, either by detecting keywords such as "viagra" or by statistical means (content or non-content based), is very popular. Content based statistical means or detecting keywords can be very accurate when they are correctly tuned to the types of legitimate email that an individual gets, but they can also make mistakes such as detecting the keyword "cialis" in the word "specialist" (see also Internet censorship: Over- and under-blocking). The content also doesn't determine whether the email was either unsolicited or bulk, the two key features of spam. So, if a friend sends you a joke that mentions "viagra", content filters can easily mark it as being spam even though it is neither unsolicited nor sent in bulk. Non-content base statistical means can help lower false positives because it looks at statistical means vs. blocking based on content/keywords. Therefore, you will be able to receive the friend who sends you a joke that mentions "viagra".
The most popular DNSBLs (DNS Blacklists) are lists of IP addresses of known spammers, known open relays, known proxy servers, compromised “zombie” spammers, as well as hosts on the internet that shouldn’t be sending external emails, such as the end-user address space of a consumer ISP. These are known as “Dial Up Lists”, from the time when end users had to dial up to the internet with a modem and a phone line.
Spamtraps are often email addresses that were never valid or have been invalid for a long time that are used to collect spam. An effective spamtrap is not announced and is only found by dictionary attacks or by pulling addresses off hidden webpages. For a spamtrap to remain effective the address must never be given to anyone. Some black lists, such as spamcop, use spamtraps to catch spammers and blacklist them.
Enforcing technical requirements of the Simple Mail Transfer Protocol (SMTP) can be used to block mail coming from systems that are not compliant with the RFC standards. A lot of spammers use poorly written software or are unable to comply with the standards because they do not have legitimate control of the computer sending spam (zombie computer). So by setting restrictions on the mail transfer agent (MTA) a mail administrator can reduce spam significantly, such as by enforcing the correct fall back of Mail eXchange (MX) records in the Domain Name System, or the correct handling of delays (Teergrube).
There are a number of techniques that individuals can use to restrict the availability of their e-mail addresses, reducing or preventing their attractiveness to spam.
Sharing an email address only among a limited group of correspondents is one way to limit spam. This method relies on the discretion of all members of the group, as disclosing email addresses outside the group circumvents the trust relationship of the group. For this reason, forwarding messages to recipients who don't know one another should be avoided. When it is absolutely necessary to forward messages to recipients who don't know one another, it is good practice to list the recipient names all after "bcc:" instead of after "to:". This practice avoids the scenario where unscrupulous recipients might compile a list of email addresses for spamming purposes. This practice also reduces the risk of the address being distributed by computers affected with email address harvesting malware. However, once the privacy of the email address is lost by divulgence, it cannot likely be regained.
Posting anonymously, or with a fake name and address, is one way to avoid e-mail address harvesting, but users should ensure that the fake address is not valid. Users who want to receive legitimate email regarding their posts or Web sites can alter their addresses so humans can figure out but spammers cannot. For instance, joe@example.com might post as joeNOS@PAM.invalid.example.com. Address munging, however, can cause legitimate replies to be lost. If it's not the user's valid address, it has to be truly invalid, otherwise someone or some server will still get the spam for it. Other ways use transparent address munging to avoid this by allowing users to see the actual address but obfuscate it from automated email harvesters with methods such as displaying all or part of the e-mail address on a web page as an image, a text logo shrunken to normal size using in-line CSS, or as jumbled text with the order of characters restored using CSS.
Spammers often regard responses to their messages—even responses like "Don't spam me"—as confirmation that an email address is valid. Likewise, many spam messages contain Web links or addresses which the user is directed to follow to be removed from the spammer's mailing list. In several cases, spam-fighters have tested these links, confirming they do not lead to the recipient address's removal—if anything, they lead to more spam. This removal request of filing a complaint may get the address list washed. To lower complaints so the spammer can stay active before having to acquire new accounts and/or internet provider.
