SMTP 250 OK Does Not Guarantee Delivery, Developer's 50-Email Test Reveals

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A developer running a 50-address email campaign on July 15 received a 250 OK response from every recipient server during SMTP handshakes, yet 12 emails — a 24% failure rate — bounced hours later. The SMTP 250 OK status only confirms that a server accepted the message envelope, not that the target mailbox exists or will deliver the message to a real user. Running the same list through a deeper email validation API exposed hidden issues including role accounts, catch-all domains, greylisted servers, and addresses tied to hundreds of known data breaches. A test with the address test@gmail.com illustrated the gap: it scored 75 out of 100 and passed basic checks, but carried a role-account flag and 579 breach records that SMTP alone never surfaced. The findings suggest that developers relying solely on SMTP handshakes for email validation risk significant bounce rates and should use multi-factor validators that check MX behavior, breach history, and account type.

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