In short, it is the email naughty list. And sometimes good emails end up on it.

This blog, Oh No, You've Been (Email) Blacklisted! Now What?, does a good job of explaining what blacklisting is, how it works and suggests ways to have email addresses removed from the lists.

You might be asking this question because our DOTS Email Valdition product returned Warning Code, '16', which means the email address belongs to a 'BlacklistedDomain'.