RFC 5322 is the internet standard published by the IETF that defines the format and syntax rules for email addresses and message headers. It replaced the earlier RFC 2822 and RFC 822 standards and is still the reference every modern email client and server implements against.

Can an email address be syntactically valid but still not deliverable?

Yes, and this is one of the most common mix-ups. Format validation only checks the structure — the @ symbol, valid characters, dot placement, and a real-looking TLD. It cannot confirm the mailbox exists, that the domain accepts mail, or that the server is currently online.

Is it safe to test real email addresses with this validator?

Yes. Validation runs entirely in your browser via JavaScript — the address you type is never transmitted to a server, stored, or shared with any third party. That makes it safe for customer lists and business contacts, not just test data.

Why does my email look correct but the checker says it's invalid?

The usual culprits are invisible whitespace from copy-pasting, consecutive dots in the local part or domain, a special character that RFC 5322 doesn't allow, an unrecognized TLD, or a local part longer than 64 characters. Retyping the address manually clears most of these.

What counts as a disposable email address?

A disposable address comes from a temporary-inbox provider such as Mailinator, Guerrilla Mail, or Yopmail, designed to receive one message and then expire. This validator flags known disposable domains as a warning rather than a hard failure, since the format itself is often still valid.

Email Syntax Validator

Validate any email address format instantly against RFC 5322 rules — local part, @ symbol, domain, and TLD. Free, browser-only, private.

Updated May 2026

Email Address to Validate

How to validate an email address

Four checks, one clear verdict

1. Type or paste an address

Drop it into the input field — works with any email format, including international domains.

2. Click Validate Email

Or press Enter — the result appears instantly, no page reload.

3. Review the four criteria

Format, Domain Syntax, Real Provider, and TLD — see exactly which one passed or failed.

What this email syntax checker does

Full RFC 5322 format validation

Four detailed criteria

Format, domain syntax, real provider, and TLD — each one evaluated and reported separately.

Metadata breakdown

Local part, domain name, TLD, and a provider reputation score from 0 to 10.

100% in your browser

No address is ever sent to a server — safe for sensitive business or customer contacts.

Specific error detection

Identifies exactly which RFC 5322 rule was broken, not just a generic "invalid" message.

Disposable domain detection

Flags addresses from throwaway providers like Mailinator or Guerrilla Mail as a warning, not a hard fail.

Common validation errors

What actually breaks a format check

Example

Issue

usernamedomain.com

Missing @ symbol

user..name@domain.com

Consecutive dots are invalid

user@domain.c0m

TLD must contain letters only

.user@domain.com

Local part can't start with a dot

When you'll reach for this tool

The scenarios that come up most

Form validation testing

QA testers throw edge cases at signup, login, and API endpoints before a release — malformed input, unicode, oversized local parts.

List hygiene before a send

Catch syntax errors in an imported CSV before it ever reaches your email marketing platform, saving bounce-rate headaches.

Debugging a custom regex

Compare your own validation pattern's output against this checker to spot where it disagrees with the RFC 5322 spec.

Learning RFC 5322 hands-on

Try different formats interactively and see, criterion by criterion, why each one passes or fails.

Getting an unexpected result? Start here

Invisible whitespace from pasting

Common when copying from a document or spreadsheet — type the address manually to rule out hidden spaces or line breaks.

Assuming "valid" means "exists"

A passing format check does not confirm the mailbox actually exists — it only confirms the syntax is correct.

Unusual TLD not recognized

Very new or uncommon top-level domains may look wrong at a glance — cross-check against IANA's official TLD list.

Why use this email validator

Unlike paid verification services that charge per credit for something you can check for free, this validator focuses on what's actually knowable client-side: whether the format is correct per RFC 5322.

The metadata panel breaks the address down into local part, domain, TLD, and provider reputation — useful for pinpointing exactly where a signup form is rejecting emails that should be valid.

Email validator vs. email verifier

Two different operations that get confused constantly

Validator (this tool)

Verifier

What it checks

format/syntax (RFC 5322)

whether the mailbox actually exists

Needs a network call

yes — MX lookup + SMTP handshake

Cost

free

usually paid per credit

Privacy

100% local, nothing leaves the browser

sends data to an external service

Frequently asked questions

A validator checks that an address is correctly formatted per RFC 5322 — syntax only, no network connection involved. A verifier goes further: it looks up MX records and attempts an SMTP handshake to confirm the mailbox actually exists. Validation is free and stays private; verification is usually a paid service since it depends on external mail servers.

References

RFC 5322 — Internet Message Format

The authoritative IETF standard defining valid email address syntax.

MDN — input type="email"

How browsers natively validate email input, and how that relates to RFC 5322.