Why a misconfigured SMTP greeting can quietly harm your email deliverability

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When one mail server connects to another, it identifies itself using either the legacy HELO or the modern EHLO greeting, which is part of Extended SMTP. Receiving servers treat this greeting as an early identity signal, and values like 'localhost' or internal hostnames that cannot be resolved through public DNS raise red flags. Best practice requires a forward-confirmed reverse DNS setup, where the sending IP's PTR record points to a fully qualified domain name that resolves back to the same IP, matching the EHLO value. A mismatch between these identifiers rarely causes outright rejection on its own, but can compound other trust issues and nudge messages toward spam folders. Correctly configuring EHLO and PTR records does not replace SPF, DKIM, and DMARC — both layers must be properly set up for reliable email delivery.

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