How can I fix the error: Client host [XXX] blocked using zen.spamhaus.org; Error: open resolver;

SpamHaus RBL is blocking all email if your server is using an open resolver. For example, public DNS resolvers like Google, Quad9, Cloudflare DNS or via any DNS server that is attempting a high volume of queries against SpamHaus without being registered with them. The SpamHaus RBL no longer accepts RBL DNS queries from any public resolver, including if you are using it as a forwarder. The mail logs will indicate the issue like this:

Client host [XXX] blocked using zen.spamhaus.org; Error: open resolver; https://www.spamhaus.org/returnc/pub/203.0.113.1

How To Fix

There are a number of options:

1. Replace any public DNS servers that your server is currently using with a local one, or your ISPs.
2. If you are using a local DNS resolver, remove the public DNS resolvers as forwarders.
3. As a last resort, you can remove zen.spamhaus.org from Tools and Settings -> Mail Server Settings -> DNS zones for DNSBL service However, Spamhaus is a very good RBL and removing may lead to an increase in spam. This is not an ideal solution, but will get email working again.

  • zen.spamhaus.org, open resolver
  • 0 Users Found This Useful
Was this answer helpful?

Related Articles

How do I fix the error: Mysql Server has gone away?

When looking at the mail log of the server you see this log entry from Amavis: Aug 3 01:00:20...

What changes does Warden make to the postfix configuration files to enable Amavis?

Important Warden will configure postfix differently depending if you selected to install the...

How hard is it to uninstall if I do not require Warden Anti-spam and Virus Protection any more?

Uninstalling Warden is as easy as going to Extensions -> My Extensions -> Warden Anti-spam...

How can I fix the error: host 127.0.0.1[127.0.0.1] said: 552 5.3.4 Declared message size (1208029 B) exceeds size limit for recipient?

You can set the message size limit for Amavis under Settings -> Policy Settings -> Misc...

How do I fix the error: Unit is masked. when trying to start SpamAssassin?

You should not try to start the SpamAssassin service. Warden disables (masks) the SpamAssassin...