Free Email Syntax Validator

01 · Definition

What is email syntax validator?

An email syntax validator checks whether an email address is structured correctly per RFC 5321 and RFC 5322 — the two specifications that define what a valid mailbox address looks like. The check runs purely on the string itself, looking at the local part (before the @), the separator, and the domain. It does not connect to a mail server, so it cannot tell you whether the mailbox actually exists or accepts mail.

Think of the tool above as a pure email format checker rather than a full email address validator. It runs the input against the strict RFC 5321/5322 format rules and returns a single verdict: valid or invalid. A full email address validator would add an MX lookup and an SMTP handshake on top — those steps are slower, cost credits, and only make sense once the syntax pass has filtered the obvious junk. Read the in-depth email syntax validation guide for the RFC rules behind each check and the most common reasons addresses fail.

02 · Process

How it works

1. 1Parse the input.Split the string at the last @ into a local part and a domain part. If there is no @, or more than one outside a quoted form, the format is invalid.
2. 2Validate the local part.RFC 5321 caps it at 64 octets, allows letters, digits, and a small set of punctuation, and forbids leading or trailing dots and consecutive dots in the unquoted form.
3. 3Validate the domain.The domain must be a fully-qualified hostname or an IP literal in brackets. Each label is at most 63 octets, the full domain at most 253, and at least one dot is required.
4. 4Return the verdict.If every rule passes, the address is syntactically valid. Anything else fails — and the tool reports Invalid so you can drop it from the list.

03 · Risk

Why it matters

Syntax is the cheapest validation step you can run, and it filters out the worst category of bad addresses before they cost you anything. Sending to a malformed address generates a hard bounce on the first attempt, hits your sender reputation, and increases the chance future legitimate mail lands in spam.

Treat syntax validation as the front door of your email hygiene workflow. Catch the obvious failures here at zero cost, then route surviving addresses through provider lookup, MX resolution, and SMTP verification only when it makes sense to spend a credit on them.

04 · Use cases

Common ways to use this tool

• Signup form gating. Reject malformed addresses at form submission and ask the user to retype before they leave the page.
• List import sanity check. Run new contact files through a syntax pass before you upload them to your ESP or CRM.
• Pre-campaign cleaning. Strip syntax failures from any list older than three months before the next send.
• Lead-form abuse triage. Spot junk submissions like aaa@aaa or ..@example that bots and bored testers leave behind.
• Migration audits. When moving lists between platforms, syntax-check the export so the new platform doesn't reject the import wholesale.

05 · Interpretation

What to check in the result

Five patterns account for almost every syntax failure on a typical exported list:

• Missing or duplicated @. Inputs like usercompany.com or user@@example.com always fail and indicate either a typo or an automated paste error.
• Empty local part. @example.com with nothing before the @ is invalid. Common in bot signups and copy-paste mishaps.
• Domain without a dot. user@localhost is technically valid for internal mail but always invalid for public delivery — catch and reject these for marketing lists.
• Leading or trailing dot. .user@example.com and user.@example.com look harmless but break RFC 5321 unquoted-local-part rules.
• Consecutive dots. john..doe@example.com fails the unquoted-form rules. The most common typo on lists exported from spreadsheets.