Enter an address to check its format piece by piece, and whether the domain is set up to receive mail at all.
Validation happens in two layers. The syntax layer runs locally and checks the parts that decide whether an address is even well-formed: exactly one @, a valid local part (length, allowed characters, no leading/trailing or doubled dots), and a valid domain (labels, hyphens, a real top-level ending). The second layer asks public DNS whether the domain publishes MX records, the records that say where its mail is delivered. A domain with no MX records cannot receive mail at that address in the normal way.
What no validator can check is whether a specific mailbox exists, mail servers deliberately do not answer that question (accepting the message then bouncing, or accepting silently, are both common anti-spam behaviours). Treat a pass here as 'well-formed and the domain receives mail', not 'this inbox is real'. The local part alone is sent nowhere: only the domain is queried against public DNS.
Can this tell me if a mailbox exists? No, and be suspicious of any free tool that claims otherwise. Servers hide mailbox existence to fight spam; only sending a message and watching for a bounce reveals it, and even that is unreliable.
What does 'no MX records' mean? The domain has not told the world where to deliver its mail. Mail to it will fail or fall back to the domain's own address record in rare legacy setups. Most legitimate mail domains publish MX records.
Is the address stored or added to a list? No. The format check is local, the DNS check sends only the domain (never the full address) to a public resolver, and this page has no server to store anything.