Donaldson R F Email Syntax Validator
Check every address in a list for correct format — and see exactly what is wrong with the ones that fail.
What is checked
This checks the format of an address, not whether the mailbox exists: one @, a name part of up to 64 characters using letters, digits and the usual symbols (. _ % + - and others) without spaces, leading or trailing dots or double dots, and a domain with at least one dot, labels of up to 63 characters that do not start or end with a hyphen, and an ending of two or more letters (not all digits). Addresses with non-English letters (internationalised addresses) are reported as such rather than accepted.
A valid format is not proof that mail will arrive: to reduce bounces also remove throw-away and role addresses (disposable email detector, role-based email detector), or run everything at once with clean email list.
Frequently asked questions
Does it verify the mailbox really exists?
No. That needs a mail server connection, which this browser-only tool never makes.
Why is a quoted address like "a b"@x.com rejected?
Quoted local parts are legal but almost never used and rejected by many systems, so they are flagged.
Is my list uploaded?
No. It stays in your browser.