Glossary
Throttling and rate limiting
SMTP throttling limits connections or commands at a receiving mail server. It can interrupt verification without establishing whether the recipient mailbox exists.
Limits imposed by the receiving server
A receiving server can limit connections or commands from a source. Emvero uses SMTP_PROVIDER_THROTTLED when it classifies an observation as a destination traffic limit. This describes what happened to the attempt; it gives no answer about the requested mailbox.
SMTP_CONNECT_REFUSED means a connection could not be opened. SMTP_CONNECT_TIMEOUT means it did not open within the allowed time. Neither establishes throttling, because server availability and network conditions can cause the same failures.
A temporary SMTP reply also needs context. RFC 5321 defines 421 as service unavailable with the connection closing; the number does not identify a traffic limit. Greylisting is a separate policy for deferring unfamiliar attempts.
When Emvero pauses further attempts
After adverse outcomes, Emvero can temporarily withhold probes for a destination group. PROVIDER_CIRCUIT_OPEN records that protection mechanism. It describes Emvero’s decision to pause, without recording a new reply from the destination. Other reasons a stage cannot run appear in service-side verification limits.
How to handle unfinished or inconclusive work
SMTP_QUEUED means SMTP work is waiting or in progress. If the response is pending, retrieve the same check after retry_after_ms. A completed inconclusive check returns unknown with allow_with_email_confirmation when no other evidence changes the decision. The verification result guide explains these fields.
A later check may add evidence after a temporary problem clears, but may use cached evidence or meet another refusal. At signup, your application can send confirmation to establish access; Emvero sends no email. For a list, retain the reason while deciding whether to hold or recheck the address. See unknown versus undeliverable for handling a check that could not reach a conclusion.
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_PROVIDER_ THROTTLED - Provider throttled the probe
PROVIDER_CIRCUIT_ OPEN - Emvero temporarily paused probes for the destination group; no new recipient reply
SMTP_CONNECT_ REFUSED - Mail server refused the connection
SMTP_CONNECT_ TIMEOUT - SMTP connection did not open before the timeout
SMTP_QUEUED - SMTP work is waiting or in progress