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

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 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.

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