One solution to the spam problem is the use of challenge-response. With this system, you maintain a "whitelist" of senders who you always want to allow email from. When mail arrives from anyone who is not on the whitelist, before it is actually delivered to you the following happens

• A "challenge" message is sent to the sender, asking them to reply in a specific manner. This may involve their including a specific word in the subject line of their reply, going to a Web site and entering information there, or any number of other mechanisms.
• If they reply correctly (the "response"), their original message is then delivered to you.
• You can then add the sender to your whitelist, or not.

The idea is that spammers will not be able to respond to the challenge, since it's a manual operation, and their mail lists often include tens of thousands to millions of addresses. This mechanism works well enough, but has some serious drawbacks for some users.

• If you ever shop on the Web, you may not get order confirmations and the like, since those are usually sent by an automated system which is unable to handle the required response. You could get around this by adding the sender's address to your whitelist, but you'd need to know ahead of time what the automated sender's address is: orders@xxxxx.com, customerservice@xxxxx.com, orderconfirm@xxxxx.com, etc. Since you don't know ahead of time what address to add to your whitelist, the automated sender will be challenged, but because the sender is an automated system, it can't respond.
• If you need to receive email from the general public (as in a business), you may lose some of it, since some senders may not understand, or may resent, the challenge-response idea, and won't respond.
• Senders on your whitelist who change their email address can't use their new address to notify you, without going through the challenge-response mechanism (since their new email address isn't yet in your whitelist). If they don't understand what's required, you won't be notified of their new address, or receive any other email from them until they notify you of their new address by some other means and you update your whitelist.

If you only get mail from friends and family then this system may be worth looking into, although other measures mentioned on our spam pages will probably work at least as well, and be easier to set up and use. Specifically, get a hard-to-guess email address and give it only to family and friends.

A few challenge-response services are

• SpamArrest
• Domain-DNS.com