A blacklist is a collection of domain names and email addresses that have been flagged as potentially dangerous. This list includes IP addresses and domains that have been linked to spam.

The majority of internet service providers and free mailbox providers use these lists to protect themselves against spam.

This might be as a result of indulging in highly frowned-upon methods and behaviors, such as buying email address lists in order to significantly increase their numbers (more on this later) or sending unsolicited or excessive emails.