Glossary
SMTP handshake
The conversation between two mail servers that establishes whether a recipient will be accepted.
SMTP is a conversation, and verification listens to the part of it that decides whether a recipient exists. The exchange opens with a greeting, the sender identifies itself with EHLO, names the envelope sender with MAIL FROM, and then names the recipient with RCPT TO. That last command carries the answer: the receiving server either accepts the recipient or refuses it, and says which in a numeric code.
Verification stops there. It never issues DATA, so no message body is ever transmitted and nothing is ever delivered — the connection closes once the recipient has been accepted or refused. This is the whole reason a mailbox can be checked without emailing the person who owns it.
The answer is not always a clean yes or no. Some servers accept every recipient by policy and decide later. Some return a temporary code on first contact. Some rate-limit probes from senders they do not recognise, or refuse them outright, in which case the honest result is that the question could not be asked.
That is why a result carries reason codes rather than a single verdict. SMTP_RCPT_ACCEPTED means the recipient was accepted at the door; SMTP_MAILBOX_NOT_FOUND means it was refused; SMTP_GREYLISTED means we were asked to come back. The verdict is a reading of those, and the codes stay in the response so that the reading can be checked.
What the service returns
Reason codes from the closed vocabulary in the API response. These are the exact strings an integration writes an if against.
SMTP_RCPT_ACCEPTED | Recipient accepted |
SMTP_MAILBOX_NOT_FOUND | Mailbox not found |
SMTP_GREYLISTED | Greylisted, a retry is required |
SMTP_CONNECT_TIMEOUT | Mail server did not answer in time |
SMTP_PROBE_SKIPPED_PROVIDER_POLICY | Probe skipped by provider policy |
Related terms
Greylisting
A spam defence that temporarily rejects the first delivery attempt from an unknown sender and accepts the retry.
Catch-all domain
A domain whose mail server accepts every address at the domain, whether or not the mailbox exists.
MX record
The DNS record that says which server accepts mail for a domain.
See it on a real address.
The free checker returns the whole result — every stage, every reason code, nothing held back.