Glossary
SMTP handshake
An SMTP handshake is the opening exchange between a client and a receiving mail server. A verifier can use it to inspect recipient acceptance before sending any message content.
The commands before message content
After connecting, an SMTP client reads the receiving server’s greeting and introduces itself with EHLO. It supplies an envelope sender with MAIL FROM, then names a recipient with RCPT TO. The server replies to each command. RFC 5321 defines this mail transaction.
The server may advertise STARTTLS before the sender command. A client that upgrades the connection to TLS sends EHLO again, following RFC 3207.
How Emvero uses the exchange
A full check can inspect the recipient reply and test whether the domain accepts arbitrary recipients. Emvero never issues DATA, the command that starts message content, so it sends no email. It may use RSET and QUIT to finish the session. A domain check opens no SMTP session.
Even a full check can finish without a handshake if domain evidence already determines the result. Provider policy can also omit the probe, recorded as SMTP_PROBE_SKIPPED_PROVIDER_POLICY.
What recipient evidence can establish
SMTP_RCPT_ACCEPTED records acceptance at the recipient step and can support deliverable. A catch-all domain can accept names with no mailbox, however; that finding makes the result risky. Acceptance cannot establish ownership, future delivery or inbox placement.
SMTP_GREYLISTED and SMTP_CONNECT_TIMEOUT leave the mailbox question unresolved. A negative reply may also reflect sender policy or reputation. The current policy converts even an observation classified as SMTP_MAILBOX_NOT_FOUND to SMTP_RECIPIENT_REJECTED_AMBIGUOUS with unknown. SMTP_FORWARDING_INDICATED is inconclusive too.
The verification process guide places SMTP within the complete check. When using the email verifier, read status together with reason_codes to see which evidence supports the result.
Related reason codes
The API uses these identifiers to explain a check. The notes identify values that the current verification policy or probe does not return.
SMTP_RCPT_ ACCEPTED - Recipient accepted
SMTP_RECIPIENT_ REJECTED_ AMBIGUOUS - Recipient refused for an ambiguous reason
SMTP_GREYLISTED - Temporary refusal classified as greylisting
SMTP_CONNECT_ TIMEOUT - SMTP connection did not open before the timeout
SMTP_PROBE_ SKIPPED_ PROVIDER_ POLICY - Probe skipped by provider policy
SMTP_FORWARDING_ INDICATED - Server reply indicated forwarding; recipient evidence remains inconclusive