"The blacklist check will test a mail server IP address against over 100 DNS based email blacklists. (Commonly called Realtime blacklist, DNSBL or RBL).  If your mail server has been blacklisted, some email you send may not be delivered.  Email blacklists are a common way of reducing spam.  If you don't know your mail server's address, start with a MX Lookup. [...] "

 https://mxtoolbox.com/blacklists.aspx

(dostęp z 2.08.2023)