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
- 01
The receiver returns a temporary response during connection, recipient, or message delivery.
- 02
The sending system queues the message and schedules a later attempt.
- 03
Retries use backoff and stop after an expiry window.
- 04
Persistent failures are classified and may contribute to recipient or domain-level suppression.
Example
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 receiverCommon 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.