---
title: "Hard bounce vs soft bounce: codes and verification - Emvero"
description: "Hard and soft bounces describe failed email delivery. Understand 4xx and 5xx replies, retry limits, and why they do not directly map to verification verdicts."
url: https://emvero.io/glossary/hard-vs-soft-bounce/
language: en
updated: 2026-10-07
---

All terms

Glossary

# Hard bounce and soft bounce

A hard bounce commonly describes a permanent delivery failure; a soft bounce describes a temporary one. Both concern a sending attempt and do not necessarily establish whether a mailbox exists.

## Permanent and temporary delivery failures

A bounce tells the sender that an attempted delivery failed. Sending systems commonly use “hard bounce” for a permanent failure and “soft bounce” for a temporary one. [RFC 5321](https://www.rfc-editor.org/rfc/rfc5321.html#section-4.2.1) defines a 4xx reply as a temporary command failure. A 5xx reply means the same request, repeated unchanged, is not expected to succeed.

A busy server or [greylisting](https://emvero.io/glossary/greylisting/) can cause a temporary refusal. A missing mailbox can cause a permanent refusal, but so can a [sender policy restriction](https://emvero.io/glossary/policy-rejection/). The reply class cannot establish the cause or show that delivery will remain impossible forever.

## What happens when retries end

A sending system may queue a message and retry after a temporary problem. If it gives up before the problem clears, the delivery has failed. The cause can still be temporary: [RFC 3464](https://www.rfc-editor.org/rfc/rfc3464.html#section-2.3.3) permits a final failed delivery notification with a 4.x.x status. Exhausting retries does not turn the error into a 5.x.x error.

## Reading a verification result

Emvero sends no verification email and stops before the message-content stage. It reports evidence from a check, without attempting message delivery. A later email may encounter different sender, content or policy conditions.

Under the current policy, a negative SMTP reply does not necessarily produce `undeliverable`. An observation classified as `SMTP_MAILBOX_NOT_FOUND` becomes `unknown` with `SMTP_RECIPIENT_REJECTED_AMBIGUOUS`. Disabled-mailbox and other mailbox-status observations can produce `risky`. Deterministic syntax or domain failures can establish undeliverability; temporary SMTP failures leave the answer unresolved.

Read the recommended action and reasons in the [verification result](https://emvero.io/blog/reading-a-verification-result/) before applying your sending rule. Consider address attributes separately. A [role account](https://emvero.io/glossary/role-account/), for example, does not predict a bounce merely because it uses a function name.

## 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_MAILBOX_NOT_FOUND`

Current policy reports this observation as SMTP_RECIPIENT_REJECTED_AMBIGUOUS

`SMTP_MAILBOX_DISABLED`

Mailbox disabled

`SMTP_TEMPORARY_FAILURE`

Temporary failure at the mail server

`SMTP_GREYLISTED`

Temporary refusal classified as greylisting

## 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.

### Ambiguous rejection

A recipient refusal can hide mailbox information or reflect server policy. A permanent-looking reply need not justify rejection.

## Compared side by side

### Email verification vs list hygiene

Technical address evidence and ongoing subscription, suppression and engagement records answer different sending questions.

### Unknown vs undeliverable

Separate inconclusive evidence from a permanent technical failure, then choose whether to retry, confirm or reject.

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

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