Whitelists are quite possibly the single most effective form of spam protection available on the market today. They are as close as you can get to totally eliminating spam from your inbox.
A whitelist is a database of trusted email addresses, IP addresses and domains. To build the list, each one of these trusted sources is manually added to the whitelist. Only email from a whitelisted source is delivered to the user’s inbox. In fact, whitelists are so effective, that the catch-rate for spam is almost 100%.
However, the efficiency of whitelists comes at a price, because it produces a large number of false positives. This means that a lot of legitimate email goes undelivered. To deal with this problem, a challenge-response technique is often instituted.
When an email from an unknown source is received, the system will respond automatically, sending a “challenge” back to the sender. This challenge may require the sender to answer certain questions, or decipher an image that displays a series of letters and numbers. This image can only be deciphered by a human, and not by spamming software. Once this is successfully done, the email is allowed to go through the system to the inbox. The sender is also added to the whitelist. The challenge-response methodology uses a combination of human judgment and software technology to determine which email to let through and which to block.
The advantage of this method is that it is not worth it to spammers to wade through all the challenge-response emails and respond to them. They are more likely to remove the email address from their lists and go after other addresses that do not have such requirements. However, the inconvenience of having to register to send email to the whitelist user may discourage legitimate email senders from following through.
