Discussion: Validating email addresses: what a syntax check can and cannot tell you
Entries
Two details on lengths and scripts. The commonly quoted maximum of 254 characters for a whole address follows from RFC 5321's 256-octet path limit minus the enclosing angle brackets, and RFC 3696's erratum records the same arithmetic; the HTML standard's grammar enforces the 63-character label limit but not the 254 total, so the length check has to be added by hand. For internationalised domains, the HTML standard tells user agents to convert the domain part of a `type=email` value to ASCII (the IDNA 'domain to ASCII' algorithm), so a form may submit `user@xn--...` for a domain the user typed in Unicode, and the server should accept either spelling; the local part is never converted, which is why an SMTPUTF8 decision cannot be delegated to the browser. Python's `email-validator` package implements the syntax rules, the optional DNS check and the IDNA conversion in one call and reports which of the three failed.
'Check that the domain has an MX record (or an A/AAAA fallback)' tests the wrong predicate. RFC 7505 defines the null MX: a domain that publishes a single `MX 0 .` record declares that it accepts no mail, and a sending MTA must treat addresses there as permanently undeliverable. A signup check that only asks 'does an MX record exist' accepts exactly those domains, which are the ones whose owners went to the trouble of saying no, and a domain with no MX but an A record is, per RFC 5321, deliverable via the A record, so absence of MX is not the failure either. The check that matches the standards is: resolve MX; if the only record is `0 .`, reject with a clear message; if there are MX records, accept; if there are none, fall back to A/AAAA and accept if either exists; and treat resolver errors as 'ask the user to check', as the article says. Without the null-MX branch, the DNS lookup gives false confidence for the one case the DNS was designed to express.
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