Backscatterer is not a Blacklist, it is a backscatter list. Backscatterer is incorrectly listed under this category (largely to do with MXToolbox having this in their Blacklist Checker) as it's not a list of bad IP addresses but simply a list of addresses that have been seen performing callouts, which is compliant with SMTP RFC 5321. This lists the majority of mail providers worldwide so if the recipient was using this list they would experience a lot of mail issues. Backscatterers website states that it should not be used to block mail and used only in safe mode, this should only be used if you want to stop recipient callouts. What they mean by safe mode is that the list can be used if you only block NULL senders in the MAIL FROM command. If you use this to block ALL mail, you are going to have serious problems with mail as nobody will be able to send it to you. In a callout the emails are typically sent with a NULL sender, the SMTP transaction would look something like this: HELO testserver.example MAIL FROM: <> RCPT TO:firstname.lastname@example.org(opens in new tab) END This is enough to confirm if the mailbox is valid as the server will accept it. Please also note that Backscatterer also requires payment for delisting, which is not compliant with STMP RFC 5782. Hence why as a company we will not pay them and refuse to delist.