---
title: "SMTP throttling during email verification - Emvero"
description: "Read SMTP throttling, connection failures, circuit protection and queued checks. Learn when to poll or check later without treating limits as a bad mailbox."
url: https://emvero.io/glossary/smtp-throttling/
language: en
updated: 2026-10-08
---

All terms

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](https://www.rfc-editor.org/rfc/rfc5321.html#section-4.2.3) defines `421` as service unavailable with the connection closing; the number does not identify a traffic limit. [Greylisting](https://emvero.io/glossary/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](https://emvero.io/glossary/our-own-failures/).

## 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](https://emvero.io/blog/reading-a-verification-result/) 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](https://emvero.io/compare/unknown-kept-apart/) 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

## Related terms

### Greylisting

A temporary SMTP refusal asks the sender to try again. A later attempt may provide more evidence.

### SMTP handshake

The greeting and commands used to ask a mail server about a recipient, stopping before message content is sent.

[All terms](https://emvero.io/glossary/)

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