Email Syntax Validator
Check an email address against the actual RFC rules — and see what syntax cannot tell you.
Email address
How this lookup works
The address is split on the last @ and each half checked against the RFC 5321 rules: length limits of 64 characters for the mailbox and 254 for the whole path, the permitted character set for an unquoted local part, and the label rules for the domain. The forms it deliberately accepts are as important as those it rejects — an apostrophe in a surname, a plus tag, a quoted local part containing an @ are all legal and are all commonly rejected by hand-written validation, which is how a signup form turns away a real customer with an unusual name. What it will not do is claim more than syntax: whether a mailbox exists is knowable only by the receiving server.
What the results mean
- Syntax valid
- Well-formed under RFC 5321. Nothing more than that.
- Mailbox exists
- Not checkable, by anyone but the receiving server.
- Quoted local part
- Legal and almost never seen. Many systems reject it regardless of the spec.
- Length limits
- 64 characters before the @, 254 for the whole address.
Common problems and fixes
- It accepts an address I think is wrong
- Check which rule you expected. Apostrophes, plus tags and long TLDs are all legal, and rejecting them is the most common validation bug.
- Can I check whether the mailbox exists?
- Not from a page, and not reliably from anywhere. It requires attempting a delivery, which is what list-cleaning services do and what gets senders blocked.
- My form rejects addresses this accepts
- Your validation is stricter than the specification. That is a choice with a cost — the customers it turns away are disproportionately those with unusual names.
Frequently asked questions
Can you verify an email address exists?
No page can. Existence is known only to the receiving server, and asking means attempting a delivery. Services that claim to verify are doing exactly that, which is why using them can harm your sending reputation.
Is user+tag@example.com valid?
Yes, entirely. Plus addressing is legal syntax and widely supported. Rejecting it is one of the most common validation mistakes and it frustrates exactly the users who care about managing their mail.
What is the maximum length?
64 characters for the mailbox name, 255 for the domain, and 254 for the whole address as a delivery path. Longer is not deliverable.
Put this on your own site
Free to embed, no attribution required beyond the source link the frame carries itself. It runs entirely in your visitor's browser, sets no cookies and loads no third-party script.
<iframe src="https://runthetests.com/embed/email-validator/" width="100%" height="560" style="border:1px solid #e5e5e5;border-radius:8px" title="Email Syntax Validator" loading="lazy"></iframe>
Preview it at https://runthetests.com/embed/email-validator/. Embedded pages are marked noindex, so yours stays the canonical copy — not this one.