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The website may be blacklisted if the site owner decides to send emails to promote a new product to thousands of people. If one or more email recipients mark it as spam, the e-mail provider may put the website in blacklist. In another scenario, if you see that your regular visitors have turned off and traffic to your site has stopped; One reason may be that your site is blacklisted.
Also, if someone suggests selling you the domain; You need to check and see if this domain is blacklisted or not.