Deliverability
What is Blacklist (DNSBL / Blocklist)?
A published list of IP addresses or domains known for spam that mail servers consult to block or filter incoming messages.
A blacklist, increasingly called a blocklist, is a database of IP addresses, domains, or URLs flagged as sources of spam or abuse. Receiving mail servers query these lists during the SMTP conversation and may reject, defer, or spam-folder mail from listed senders. Major providers include Spamhaus, Barracuda, and SURBL, and listings can be triggered by spam-trap hits, high complaint volume, or sudden sending spikes.
Why it matters for cold email
A single blacklist appearance, especially on Spamhaus, can collapse cold-email delivery across many recipients at once because so many providers consume those lists. Cold senders need to monitor listings continuously and have a delisting and remediation process ready before resuming volume.
Related terms
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