Email Address Structure

The Anatomy of an Email Address

Every valid email address follows a standardized format defined by RFC 5321 and RFC 5322: [email protected]. The @ symbol separates the local part (left side) from the domain (right side). The domain consists of a domain name and a top-level domain (TLD) like .com, .org, or .io. This seemingly simple structure has surprisingly complex rules governing what's valid and what isn't.

The Local Part

The local part is the portion before the @. It can contain letters (a–z, A–Z), digits (0–9), and certain special characters: periods, hyphens, underscores, and plus signs are commonly allowed. The local part is case-insensitive in practice (though technically case-sensitive per the spec). Maximum length is 64 characters. Periods cannot appear at the start or end, and consecutive periods are not allowed. The plus sign (+) is often used for address tagging (e.g., [email protected]), which routes to the same mailbox.

The Domain Part

The domain part consists of one or more labels separated by dots. Each label can contain letters, digits, and hyphens — but cannot start or end with a hyphen. The rightmost label is the TLD (.com, .org, .net, .io, .co.uk, etc.). Maximum total domain length is 255 characters. The domain must have valid DNS records — specifically an A record and/or MX record — for email to be deliverable.

Common Syntax Errors in Email Addresses

Missing @ symbol (user.domain.com). Multiple @ symbols (user@@domain.com). Space in the address (user [email protected]). Invalid characters in the local part ([email protected] — the $ is not permitted). TLD missing or invalid (user@domain or [email protected]). Consecutive dots ([email protected]). These are the errors that syntax validation catches — and they're surprisingly common in form submissions, especially on mobile.

Syntax Validation vs. Full Verification

Syntax validation checks whether an address conforms to the structural rules described above. It's fast and can be done without any network requests. But syntax validity is only the first step — a syntactically valid address ([email protected]) can still be completely undeliverable. Full email verification, as performed by EmailVerify.io, goes further: it checks domain DNS records, MX records, and actual mailbox existence via SMTP. Syntax check is necessary but not sufficient.

International Email Addresses

Modern email standards support internationalized domain names (IDN) with non-ASCII characters, and internationalized email addresses (IEA) with non-ASCII characters in the local part. While these are valid per modern RFCs, support across email clients, servers, and verification tools varies. In practice, most business email still uses ASCII characters, and internationalized addresses are more common in consumer email in non-Latin-script languages.