What is an email syntax check?
Also called: format check, email regex
A syntax check tests whether an email address is correctly formed: one @, a local part and a domain made of permitted characters, and lengths within the limits set by the email standards. It is the first and fastest verification step, and it proves nothing about whether the mailbox exists.
The formal grammar in RFC 5322 allows far more than most people expect, including quoted local parts with spaces. In practice every verifier applies a stricter, practical rule, because addresses that use those features are vanishingly rare and often rejected by sending platforms anyway.
The length limits come from RFC 5321: up to 64 octets for the part before the @ and 255 for the domain. A regex that ignores them lets through addresses no mail server will accept.
A syntax check is the right thing to run in the browser while someone fills in a form. On its own it is not verification: nobody@realcompany.com is perfectly well formed.
Example
jane.smith@company.com passes. jane.smith.company.com (no @) and jane@@company.com fail.How SimpleVerifier handles it
Malformed addresses are returned as invalid with the reason Invalid format, before any DNS lookup or connection is made.
Go deeper
Sources
By SimpleVerifier Team · Last reviewed 2026-10-13 · All 50 glossary terms