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.
How to check if your domain is on a blacklist:
Please visit MX Toolbox Blacklist Tool and enter your domain name into the Server of Domain Field:
If your domain is not listed on any global blacklists, you will see green tick boxes all the way along the list of providers such as the below:
However, if your domain is listed on a blacklist, you will see something like the below which will list the entries for your domain:
A Blacklisted domain wll often end up in other peoples junk or spam folders. In the case where a company you are trying to send to is using a product such as Barracuda Email Security, Symantec, Mimecast etc, your email may not even pass through to their email folders.