Every Email Rejected at Once — The Open-Resolver Mistake That Makes Spamhaus Answer "No"

A presales war story about a mail server that suddenly rejected every sender on the internet — Gmail, Outlook, the bank — and the two DNS queries I now run on every mail server before anyone blames the blacklist. The message came in early in the morning. "Nobody can email us.

Source: [Dev.to](https://dev.to/daanial/every-email-rejected-at-once-the-open-resolver-mistake-that-makes-spamhaus-answer-no-1j5d)