SMTP probe
Opening a real SMTP conversation with the receiving mail server to ask whether a mailbox exists, without sending a message.
An SMTP probe connects to the domain's mail server on port 25 and walks through the start of a normal delivery: greeting, sender, then recipient. The server's answer to the recipient command reveals whether it would accept mail for that address. The conversation is then abandoned before any message body is sent, so nothing lands in an inbox.
This is the only check that tests the mailbox itself. Syntax checking catches typos, DNS and MX checks prove the domain can receive mail at all, but neither can tell you that a particular person's address still exists.
Probes are not universally possible. Many networks block outbound port 25, some providers greylist unfamiliar senders, and catch-all domains answer yes to everything. A verification service that never reports these cases is hiding them.