SMTP Verifier Blocklisted by Spamhaus Despite Never Sending a Single Email

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A developer building an email verification service discovered his server had been blocklisted by Spamhaus and rejected by iCloud, even though the server had never sent any email. The service works by initiating SMTP handshakes to check if mailboxes exist, then disconnecting before any message is transmitted. Investigation revealed the root cause was a misconfigured HELO hostname that did not match the server's PTR and A records, a mismatch that triggered Spamhaus detection during probe connections. After correcting the hostname configuration to ensure all four SMTP identity checks aligned, no new detections were recorded. The Spamhaus listing subsequently expired on its own within a few days, resolving the issue without requiring a manual delisting request.

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ProgrammingDEV Community ·

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