Engineering notes
What email validation actually does against live DNS.
How email validation actually behaves against live DNS: typo correction, RFC 7505 null MX, the SERVFAIL/NXDOMAIN split, and failing open in a signup flow.
Validation engine
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One Mail Host Serves Both gmai.com and hotmial.com
Typo domains are an unenumerable long tail, but the operators collecting mail for them reuse infrastructure, so the MX host is the tractable signal.
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Suggest the Correction Instead of Rejecting the Signup
Rejecting a mistyped address ends the session; naming the intended address recovers it, which means the interesting engineering is in the suggestion.
DNS and RFCs
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MX Validation Does Not Catch the Typos That Matter
An MX lookup accepts the four most common Gmail and Hotmail misspellings, because typo domains are registered and resolve correctly.
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Null MX, RFC 7505, and Why Reserved Domains Need Their Own Reason Code
A domain publishing MX 0 . is telling you it accepts no mail at all, and example.com does the same — but breaking your test suite is not the same problem as a bad signup.
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SERVFAIL Is Not NXDOMAIN, and Conflating Them Accepts Bad Addresses
A resolver failure and a nonexistent domain arrive as different DNS status codes and demand opposite responses: fail open on one, reject on the other.
Signup flows
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Fail Open: Email Validation Must Never Block a Signup
Treat the validator as advisory infrastructure: set a timeout, branch on the verdict, record what was actually checked, and let users through when it cannot answer.
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Validate Email Addresses From an AI Agent
An autonomous agent can obtain access, validate an address and recover from an error without a human in the loop, provided every step is a single HTTP request.
Choosing a validator
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How to Choose an Email Validation API
Accuracy percentages on vendor pages are unfalsifiable, so evaluate the response contract, the failure behaviour and the retention policy instead.
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Why nobounce Will Never Do SMTP RCPT TO Probing
The technique that promises per-mailbox certainty depends on infrastructure reputation, returns weak signals at the largest providers, and is permanently outside this product's scope.