What are blocklists (DNSBL)
Mail servers reject millions of spam messages every second. Rather than analysing every message from scratch, they check the sender's address against blacklists called DNSBLs. If your server's IP ends up on such a list, your mail will land in spam or not be delivered at all.
How an address gets listed
- A hacked website or mailbox sends spam on your behalf.
- The server has malware or an open mail relay.
- Bulk mailings without the recipients' confirmed consent.
- You were given an address that previously belonged to a spammer, or a neighbour in your subnet breaks the rules (lists such as UCEPROTECT L2 block entire ranges).
What to do if you are listed
First remove the cause: change mailbox passwords, scan your site for malicious code and limit sending from forms. Then use the “How to get delisted” link — most blocklists remove an address automatically or on request. Set up SPF, DKIM and DMARC so recipients trust your mail.
“No response” means the list did not answer in time or rate-limits queries — it does not indicate a block.