Email blocklist (DNSBL)
An email blocklist is a public list of IP addresses or domains linked to spam, which mail servers check to decide if they should reject your email.
A blocklist, also called a DNSBL (DNS-based blocklist), is a database that anyone can query through DNS. A receiving server takes the sending IP address, or a domain found in the message, and asks the list if it is known for abuse. If it is, the server may reject the mail or send it to spam. Spamhaus is the best-known operator, and it has separate lists for IP addresses and for domains.
Not all lists have the same weight. Some are used by large providers and a listing there hurts a lot. Others are small and rarely matter. Domain blocklists matter for cold email because they look at the links in your message, not only the sender.
If you are listed, stop sending, find the cause (a bad list, a hacked account, a spam trap), fix it and then follow the list's removal steps. Asking for removal without a fix leads to a new listing. Check your sending domains and links before a campaign starts, not after.
In Terlido
Terlido can check the domains you send from against public blocklists when you start an audit of your sending domains.