SPAM is the ultimate form of shotgun marketing.  Most SPAM marketers expect fewer than 1 in 5000 e-mail recipients to ever follow one of their links (and less still to actually buy the product advertised on the other side).  In order to support such an inaccurate form of marketing, spammers need to build up huge pools of e-mail addresses at very little cost.

That’s where e-mail spam-bots come in.  According to Wikipedia:

E-mail spambots harvest e-mail addresses from the Internet in order to build mailing lists for sending unsolicited e-mail, also known as spam. Such spambots are web crawlers that can gather e-mail addresses from Web sites, newsgroups, special-interest group (SIG) postings, and chat-room conversations.

Because there are only so many ways to write an e-mail address, it is fairly easy for spammers to create these programs.  Simple ones just look for strings of code that include the @ symbol.

This suggested some of the earliest anti-spambot strategies.

For example, it is possible to write code that appears one way on your browser, but another way to the people looking at the code itself.

Unfortunately, there are certain rules that coders must still follow when taking this approach.  For most approaches, the spammers needed only to write these rules into their own code, and the fix no longer worked.

So how does one hold a spam-bot at bay?

It is important to realize that no solution is perfect.  Technology is ever improving, and techniques that work today may eventually be overcome.

However, there are two underlying principles that remain the same.

1)   E-mail spam bots are used to save costs. They will only be updated to overcome a particular anti-spam strategy once its use is widespread enough to justify the extra cost and effort.

2)   Human input is in limited supply. Spammers do have tricks that let them overcome certain human-only interfaces - but they prefer to use these strategies to enter forums, blogs, and other sites where they can hit multiple eyes at once.

Therefore the best solutions are ones that take a novel and/or cutting edge approach to ensure your e-mail can only be gathered by the actions of a human.