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A website could get on a blacklist if the website owner decided to send out emails promoting a new product to thousands of people. If one or some of the receivers of the mail marks it as spam, the email service provider might place the website on a blacklist. In a different scenario, if you see that your regular site visitors have dropped off and the traffic to your site has stopped; one of the reasons could be that your website has been blacklisted. Also if someone is offering to sell a domain to you; you must check and see if that domain is blacklisted or not.
Some organizations are running the blacklist service on the internet. Email service providers check if a website is blacklisted with DNSBL or SURBL. Email service providers use these services to try and eliminate spam mail before it reaches the user. Some DNSBL service providers use a variety of criteria to list and delist website addresses. Spam filters used by email service providers use DNSBL services as the first line of defense. SURBL is not the first line of defense in checking spam mail. It contains a directory of websites that have been blacklisted for appearing in the text of spam emails. Both these methods are used by email service providers as their first line of defense against spam mail.