Developer loses IP warm-up after 24% bounce rate despite SMTP 250 OK verification

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A developer running a cold email campaign on March 14 sent 50 messages to leads that had all passed basic SMTP verification, only to see 12 bounce — a 24% failure rate. The core issue is that an SMTP 250 OK response confirms a server accepted the envelope, but does not guarantee the mailbox exists or is actively used. Using a third-party email validation API, the developer found that addresses like test@gmail.com, while syntactically valid and SMTP-friendly, are flagged as role addresses with reduced deliverability scores. The API returned additional signals — including null SMTP verification status, a deliverability score of 75, and breach history — that a raw SMTP handshake cannot provide. The incident cost the developer a fresh IP warm-up, roughly three hours of list cleanup, and required explaining the bounce spike to a client.

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