What this tool checks
Two independent things: whether the address is syntactically well-formed (a local-part@domain.tld shape), and whether that domain has MX (Mail Exchange) records — the DNS records that tell the rest of the internet which servers actually handle mail for it. An address can look perfectly valid and still be undeliverable if its domain has no MX records at all.
Why this isn't a full deliverability guarantee
Confirming a specific mailbox exists would require an SMTP handshake (connecting to the mail server and issuing a RCPT TO command) — a technique this tool deliberately avoids. Most mail servers today accept every RCPT TO command regardless of whether the mailbox exists, specifically to stop spammers from using this exact technique to harvest valid addresses. The result would be misleading at best, and running it at scale looks like abuse to the mail server being probed.
What a passing result actually tells you
A syntactically valid address on a domain with working MX records means: the address is plausible, and mail sent to it has somewhere to go. It doesn't confirm the specific mailbox is active or monitored — for that, a real confirmation email with a double opt-in click is still the only reliable method.