Before e-mail from a new address can be allowed into the inbox, the user must answer questions or type in a partially obscured code that is recognizable on normal computers, but cannot be detected by the kind of software spammers often use.

The result is a combination of computer technology and a user's own judgement to decide which users will be allowed to send e-mail to the account or not. Once the potential sender is approved, he or she can send e-mail to the account without having to re-register.

Many people prefer whitelists to spam filters and spam blockers. Spam filters are often free with any e-mail provider, and use probability formulas to calculate the likelihood that e-mails containing certain keywords will be spam.

The problem with this approach is that many spammers can get around these filters by using words not associated with spam and by intentionally misspelling keywords to fool the filter. Although filters are effective, there is usually a 10% to 20% failure rate. This may be acceptable to some, but for business owners who receive hundreds of e-mails a day, this means that at least 100 spam e-mails a day end up in the inbox.

Spam blockers offer a higher degree of protection, but also have their drawbacks. Spam blockers must be downloaded and require registration to send e-mails. All unfamiliar e-mails are sent to another file for the user to approve. Spam blockers, unlike filters, are entirely user driven, but have the disadvantage that sometime the user can be fooled concerning what is spam. Spammers often spoof e-mails that are familiar to the user to encourage the user to accept the e-mails.

Whitelists combine the best of technological ability and user judgement. The user approves a message, but the person registering is required to type in a code that would not be recognizable to those with spamming software. Blocking is not merely left to the judgement of the user or to the computer's ability to find spammers, but the combination of the two make whitelists the most effective choice for spam blocking.

Some may find whitelists to be a bit strict and inconvenient, and fear that some legitimate senders might be put off by the registration process. Others would rather take this risk than to accept spam. Once a person is registered, his or her e-mail is accepted on the site, so there is no more red tape. Another objection is that sometimes the code is difficult to read even for legitimate senders. However, a sender can always try to register again unless he or she has been blacklisted.

Although some believe that whitelists threaten to take some of the spontaneity out of e-mail, others believe that this spontaneity is not worth all of the spam. Business owners use whitelist to ensure that no valuable time is wasted sorting through hundreds of spam messages a day, and this also saves money in the long run.