SMTP 250 OK Confirms Acceptance, Not Delivery: 12 of 50 Validated Emails Bounced

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A developer ran a controlled experiment last Tuesday, sending plain-text emails to 50 addresses that had already cleared regex syntax checks and live SMTP handshakes. Despite 48 of those addresses returning a 250 OK response, 12 emails ultimately bounced after actual delivery was attempted. The findings highlight a fundamental misunderstanding in email validation pipelines: an SMTP 250 OK response only confirms that a mail server accepted the envelope, not that the message reached a real, active inbox. Testing the same batch through a third-party validation API revealed that even addresses marked as syntactically valid with confirmed MX records could return a null SMTP verification status and a deliverability score well below 100. The experiment underscores that binary checks like syntax parsing and MX lookups are insufficient proxies for true email deliverability.

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