What is the issue?
SpamHaus RBL is blocking all email if using an open resolver. For example, Google's public DNS 126.96.36.199, Cloudflare 188.8.131.52, and so on.
What causes the issue?
The SpamHaus RBL is no longer accepting RBL DNS queries from any public resolver, including if you are using it as a forwarder. The mail logs indicate the issue like this:
blocked using zen.spamhaus.org; Error: open resolver;
What is the solution?
There are a number of options:
Go to System Setup > Network > DNS Servers and replace the DNS with a local one or your ISPs.
Alternatively, if you are using a local DNS resolver, remove the public DNS resolvers as forwarders.
As a last resort, you can remove zen.spamhaus.org from System Setup > IP Controls > RBL Servers. However, Spamhaus is the premier RBL in the industry and removing it will lead to an increase in spam.
This is not a recommended solution but will return mail flow in an emergency.
A statement from SpamHaus on the use of public DNS servers can be found here.