This test will check a mail server IP address against 109 DNS based Realtime Blackhole Lists (also known as RBL, DNSBL or email blacklist).
If your mail server has been blacklisted in one of the lists, your outgoing email might be considered as SPAM.
If you don't know your mailserver's hostname, you can find it out by doing an MX Lookup on your domain (e.g. newslettertech.com). This is only valid if your outgoing and incoming mailserver are equal since the MX lookup returns your incoming mailservers but you need to enter your outgoing mailservers here.