SMTP verification
Also: SMTP probing, email verification, RCPT TO check
Checking whether an email address exists by opening a connection to the domain's mail server and asking it to accept a recipient, without sending a message.
Because RFC 5321 lets servers disable the VRFY command, verifiers usually probe with RCPT TO instead. Repeated probing is how sending IPs end up on blocklists, and a catch-all server that accepts every recipient makes the answer meaningless.