Based on DNS blacklists (DNSBL) or black lists in real time (RBL), also known as black lists of IP-addresses. This list of IP-addresses that are distributed and modified in real time using technology based on the implementation of domain name system (DNS), and initially were designed to filter spam messages to email systems.
Today, most mail servers can be configured to work with black lists DNSBL or RBL blocking the flow of spam messages made to these blacklisted IP-addresses.
Around the world, many companies have developed and maintain blacklists used with different purposes. Most of them are still designed to block spam email systems.
On this website you can to check your spam or any other IP-addresses in the most famous and most popular databases blacklists IP.
If any IP address was listed as DNSBL, which is used by some mail server, this server based on their preferences may refuse, mark as spam, or completely block messages coming from the addresses listed with.
For end-user, it could mean that he won't be able to send any e-mail message to any recipient of the corresponding mail server. Thus, if you face a problem to deliver your mail messages to some your recipients, first of all, you need to check if your IP was blacklisted with some DNSBLs or not.