Definition
A soft bounce is a temporary email delivery failure. Unlike a hard bounce (which indicates a permanent issue like an invalid address), a soft bounce occurs when the receiving server cannot deliver the message at that moment but may be able to deliver it later.
Soft bounces require different handling from hard bounces. Retrying a soft bounce is appropriate. Retrying a hard bounce wastes resources and damages sender reputation.
Soft Bounce vs Hard Bounce
| Factor | Soft Bounce | Hard Bounce |
|---|---|---|
| Cause | Temporary issue | Permanent issue |
| Examples | Mailbox full, server timeout, message too large | Invalid address, domain does not exist |
| Retry | Yes — appropriate to retry | No — remove immediately |
| Suppression | After repeated soft bounces | Immediate |
| Impact on reputation | Low if managed properly | High if not removed |
Common Soft Bounce Causes
| Cause | SMTP Code | Description |
|---|---|---|
| Mailbox full | 452, 552 | Recipient has exceeded storage limit |
| Server timeout | 421 | Receiving server temporarily unavailable |
| Rate limiting | 452 | Sending too fast for the receiving server |
| Message too large | 552 | Email exceeds the receiving servers size limit |
| Grey listing | 451 | Server temporarily rejects to verify sender |
| Connection refused | 421 | Receiving server busy or down |
How to Handle Soft Bounces
- Retry automatically: Most ESPs retry soft bounces 3-5 times over 48-72 hours.
- Track soft bounce frequency: If an address consistently soft bounces, it may indicate a permanent problem that will not resolve.
- Set a soft bounce threshold: Suppress addresses after 3-5 consecutive soft bounces. Continued sending to recurring soft bounces wastes resources.
- Monitor patterns: A sudden increase in soft bounces across your list may indicate a deliverability issue (rate limiting, blacklisting) rather than individual address problems.
Soft Bounce Benchmarks
| Metric | Healthy Range | Action Required |
|---|---|---|
| Soft bounce rate | Below 3% | Normal |
| Soft bounce rate | 3-5% | Investigate list or delivery infrastructure |
| Soft bounce rate | Above 5% | Review list sources and sending practices |
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Frequently Asked Questions
Most ESPs allow 3-5 consecutive soft bounces before automatically suppressing an address. The exact threshold depends on the cause — mailbox full may resolve, while repeated grey listing may not.
Yes. A mailbox that was temporarily full may remain full permanently, turning a soft bounce into a de facto hard bounce. This is why repeated soft bounces should eventually trigger suppression.
Soft bounces have a minimal impact on sender reputation if they are a small percentage of total sends. A high soft bounce rate (above 5%) may indicate poor list quality or sending practices and can negatively affect reputation.
Grey listing is a specific type of soft bounce where the receiving server temporarily rejects emails from unknown senders to verify the sender's identity. It typically resolves after the sender retries. Not all soft bounces are grey listing.
Not immediately. Allow retries to complete and suppress only after repeated soft bounces. Immediately removing soft bounces loses temporarily unavailable recipients who are often valuable contacts.