These are more popular among the recipients who work at home. Here is what a challenge/response filter like Spamarrest does. If you are not on the recipient’s whitelist or address book yet, then you’re considered a stranger to him. And if you’re a stranger, you could be a spammer. So, the challenge/response filter sends you an automatic reply with a question that you have to answer, or some link you have to click (this is to prove you’re a human). When you click on the link, a page opens telling you that your message was delivered to the recipient.
It’s important to remember that you have to be “whitelisted” if you want your emails to get through. So when people fill out your opt-in form on your website, ask them to “please add our email address to your address book…”. Whenever you send a newsletter, make sure the “Reply-to” email address that you use is valid, and that a human checks it after each campaign. You can expect to receive a couple auto-replies like this after every send.
Visit http://www.glocksoft.com/spamcombat/ to read more about the problem of SPAM and how to block it.