// Glossary
SMTP Check
A live handshake with a mail server that asks whether a specific mailbox would accept delivery, without sending an email.
Definition
An SMTP check verifies a mailbox by opening a connection to the recipient's mail server and beginning the delivery conversation (EHLO/HELO, MAIL FROM, RCPT TO) up to the point the server says it would accept or reject the address. It stops before sending the actual message. The server's response indicates whether the mailbox exists, though catch-all domains and greylisting can make results inconclusive.