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.XXX.XXX.XXX] blocked using zen.spamhaus.org; Error: open resolver; https://www.spamhaus.org/returnc/pub/203.0.113.1How 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 Warden -> Mail Server Settings -> DNSBLs and replace it with alternatives like b.barracudacentral.org, bl.spamcop.net, psbl.surriel.com, or spam.spamrats.com. More information about DNSBLs we recommend can be found here.
