What is DNSBL and IP black lists?
A DNS-based Blackhole List (DNSBL) or Real-time Blackhole List (RBL) are also known as IP black lists. Them are lists of IP addresses, delivered through Domain Name System (DNS) implementation, and are initially created for special purpose to help filter SPAM messages.
For today, most of mail server software could be configured to work with DNSBLs or RBLs to block spamming flow from a blacklisted IP addresses.
Currently over the world there are a lot of IP black lists, which are maintained by different companies for different special purposes. Most of the black lists are intended to solve a problem with e-mail SPAM.
At this website, you can perform blacklist check for your IP address or for any other address across the most popular and known IP blacklist databases.
If some IP address once was listed in a DNSBL which is then used by the mail server software, that server, depending on its configuration, may deny, move to SPAM or totally drop all incoming mail traffic from the blacklisted address.
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.