Opt-in email is email containing information or advertising that users explicitly request (opt) to receive. Typically, a website invites its visitors to fill out forms identifying subject or product categories that interest them and about which they are willing to receive email from anyone who might send it. The website sells the names (with explicit or implicit permission from their visitors) to a company that specialises in collecting mailing lists that represent different interests. Whenever the mailing list company sells its lists to advertisers, the website is paid a small amount for each name that it generated for the list. You can identify opt-in email because it usually starts with a statement that tells you that you have previously agreed to receive such messages.