# Soft bounce

> A temporary delivery failure that may succeed when retried under a controlled policy.

- **Category:** Deliverability
- **Updated:** July 2026
- **Canonical page:** [https://emailbump.com/glossary/soft-bounce](https://emailbump.com/glossary/soft-bounce)

## Definition

A soft bounce is a temporary failure, often represented by an SMTP 4xx response. Causes include a full mailbox, receiver outage, throttling, greylisting, temporary DNS failure, or a short-lived reputation decision.

Temporary does not mean retry forever. Senders need bounded retry schedules and escalation rules because persistent deferrals can reveal a reputation or configuration issue.

## Why it matters

Immediate suppression after one temporary failure loses valid recipients, while aggressive retries can worsen throttling and create unnecessary load. The response pattern determines the right action.

## How it works

1. The receiver returns a temporary response during connection, recipient, or message delivery.
2. The sending system queues the message and schedules a later attempt.
3. Retries use backoff and stop after an expiry window.
4. Persistent failures are classified and may contribute to recipient or domain-level suppression.

## Example: Temporary throttling

```text
421 4.7.0 Temporarily deferred due to user complaints

Class: temporary, reputation-related
Action: slow retries, inspect complaint and volume trends, do not hammer the receiver
```

## Common mistakes

- Retrying rapidly without backoff.
- Treating a full mailbox and reputation throttling as the same operational problem.
- Suppressing after one ambiguous temporary response.
- Ignoring persistent 4xx patterns because the messages have not permanently failed yet.

## Key takeaways

- Use controlled retries with an expiry window.
- Group deferrals by provider and diagnostic.
- Persistent temporary failures deserve investigation.

## Related definitions

- [Hard bounce](https://emailbump.com/glossary/hard-bounce.md) — A permanent delivery failure, commonly caused by an invalid address or a definitive policy rejection.
- [Sender reputation](https://emailbump.com/glossary/sender-reputation.md) — A receiver-specific assessment of sending identity and behavior used when filtering email.
- [IP reputation](https://emailbump.com/glossary/ip-reputation.md) — The sending history and risk signals mailbox providers associate with an IP address.
- [Suppression list](https://emailbump.com/glossary/suppression-list.md) — A durable do-not-send control containing recipients or destinations that must not receive certain email.

## Related guides

- [Gmail reputation-based bounces: diagnose, contain, and recover](https://emailbump.com/blog/gmail-reputation-bounces.md) — An incident guide to Gmail 4.7.x and 5.7.x responses, sender-level classification, safe retries, Postmaster Tools, abuse containment, and reputation recovery.
- [Email bounces: how to classify, retry, suppress, and recover](https://emailbump.com/blog/email-bounce-handling-guide.md) — A practical operating guide to SMTP failures, enhanced status codes, retries, suppression, webhooks, and the product workflows that protect sender reputation.
- [How to check your sending IP reputation in 2026](https://emailbump.com/blog/check-sending-ip-reputation.md) — A practical audit for finding your real sending IPs, reading receiver-specific signals, and fixing the behavior behind a reputation problem.
- [The email deliverability checklist for every campaign](https://emailbump.com/blog/email-deliverability-checklist.md) — A practical pre-send routine for protecting your sender reputation and reaching more inboxes.

## Primary sources

- [RFC 5321: Simple Mail Transfer Protocol](https://www.rfc-editor.org/rfc/rfc5321)