A blocklist (often called a blacklist) is a published list of IP addresses or domains associated with spam. Mail servers consult them when deciding whether to accept a message.
Two kinds
- IP blocklists list sending servers. On shared hosting, the IP is shared by accounts on the server.
- Domain blocklists list domains that appear in spam, including domains linked in message bodies.
Checking
Use a multi-blocklist lookup tool and enter:
- your domain
- the sending IP, found in the headers of a message you’ve sent (look for the first
Received:line added by your server)
Reading the results
Not every list matters. Major lists such as Spamhaus are widely used; many small lists are ignored by large providers. A listing on an obscure list that nobody checks rarely explains delivery problems.
If the domain is listed
The cause is almost always content or behaviour on your side: a compromised mailbox, a hacked website sending spam, or links to your site appearing in spam. Fix the cause, then follow the list’s delisting process. My email account is sending spam My website has been hacked: what to do
If the shared server IP is listed
Delisting the server is a platform matter, not something you can do. Open a ticket with the help desk including the list name and the bounce message. Meanwhile, make sure your own authentication is correct so your mail is judged on your domain’s reputation as well.
Bounce messages that point to a list
Rejections often name the list, for example 554 ... listed at zen.spamhaus.org. Understanding email bounce messages