What are DNSBL IP blacklists and how to avoid getting onto them?

DNSBL, or DNS Blackhole List, is a technology for publishing lists of IP addresses from which spam distribution, malware, or other types of network abuse have previously been recorded. Mail servers around the world use DNSBL databases in real-time when receiving an incoming connection. If the sender's IP address is found in one of these databases, the receiving server can instantly reject the connection or place the incoming email into the spam folder even before the message content transmission begins.

There are many different DNSBL databases, ranging from large international projects to local lists of individual email providers. Each blacklist sets its own criteria for including IP addresses and rules for excluding them. Getting onto such lists can happen due to an account compromise on your server, vulnerabilities in web mail-sending scripts, sending emails to non-existent spam trap addresses, or incorrect reverse DNS configuration.

To prevent getting onto blacklists, you need to follow a number of important mail server administration rules:

If your IP address still ends up blocked, you must promptly eliminate the cause of the incident (for example, closing a vulnerability on the website or blocking a compromised mailbox), and then submit a request for removal from the specific blacklist via the delisting procedure.