What makes an email address valid?
A valid email address has three parts: a local part, an @ symbol, and a domain. Each part has rules defined by RFC 5322, the internet standard for email format.
- Local part — before the @. Letters, numbers, and most special characters allowed. Max 64 characters.
- @ symbol — exactly one required. No more, no fewer.
- Domain — after the @. Must contain at least one dot and end with a valid TLD (e.g. .com, .org, .io). Max 253 characters.
- Total length — the full address must be 254 characters or fewer.
Format validation vs. existence check
This tool checks format — whether the email address is structurally valid according to RFC 5322. It does not check whether the address actually exists or whether the mailbox is active. For that, you would need to either send a verification email or use an SMTP probe.
- Format check (this tool) — fast, works offline, no false positives from non-existent domains
- DNS MX lookup — confirms the domain has mail servers, but not the specific address
- SMTP probe — queries the mail server directly, often blocked by providers to prevent scraping
- Verification email — the only fully reliable method; requires user action
Common use cases
- Cleaning email lists — remove obvious format errors before importing into a CRM or mailing tool
- Debugging form validation — test which addresses pass or fail your app's email regex
- CSV/export review — quickly check a batch of user-submitted addresses from a data export
- QA testing — generate edge cases and confirm your validation logic handles them correctly