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Deliverability

SMTP

SMTP is the protocol mail servers use to hand messages to each other. In verification, an SMTP probe opens a conversation with the receiving server and asks whether a specific mailbox exists, without sending anything.

The probe follows the start of a normal delivery. It connects, introduces itself, names a sender and then names the recipient. At that last step the receiving server either accepts the address or rejects it, and the probe stops there. Nothing arrives in the mailbox.

Not every server answers honestly. Some accept every recipient by design, some reject unfamiliar senders regardless of the address, and some rate-limit probes heavily. That is why a careful verifier returns three outcomes rather than two — reporting an unknowable address as valid is how a list with a clean report still bounces.

Goes deeper: Email Verification, Explained Properly

Related terms

Email verification
Email verification is checking whether an address can actually receive mail before you send to it. A full check tests the syntax, then the domain's mail records, then asks the receiving server about the specific mailbox.
MX record
An MX record is the DNS entry naming the server that handles mail for a domain. If a domain has no MX record, nothing sent to any address at that domain can be delivered.
Catch-all email
A catch-all domain accepts mail for every possible address, including ones that do not exist. Because the server says yes to anything, no verifier can confirm whether a specific mailbox there is real.
Greylisting
Greylisting is a spam defence where a server temporarily rejects mail from an unfamiliar sender and accepts it on the retry. Real mail servers retry; most spam software does not.