Every Email Rejected at Once — The Open-Resolver Mistake That Makes Spamhaus Answer “No”
One morning a cPanel server rejected almost every inbound email, including Gmail and Outlook, with “is in an RBL: Error: open resolver”. The senders were fine. The server’s own DNS resolver was forwarding to public DNS, and Spamhaus refuses those queries. Here is the cause, the fix and the two-query test.