Hard Bounce vs Soft Bounce: What They Mean and How to Handle Them
Email bounces hurt your sender reputation. Learn the difference between hard bounces and soft bounces, their subtypes, and the right way to handle each.
What is an email bounce?
An email bounce occurs when a message cannot be delivered and the receiving mail server returns a delivery failure notification (also called a Non-Delivery Report or NDR). The bounce contains an SMTP status code (like 550 or 421) and a human-readable explanation of why delivery failed.
Bounces fall into two main categories: hard bounces (permanent failures) and soft bounces (temporary failures).
Hard bounces
Hard bounces are permanent delivery failures. The email address or domain is invalid and will never accept mail. Remove hard-bounced addresses immediately and permanently — continuing to send signals poor list hygiene to mailbox providers.
Common hard bounce SMTP codes
| SMTP Code ↕ | Name ↕ | Cause ↕ | Action Required ↕ |
|---|---|---|---|
| 550 | Mailbox unavailable | Email address does not exist at this domain | Remove immediately; suppress permanently |
| 551 | User not local | Server refuses to relay; address may have moved | Remove from list |
| 552 | Exceeded storage allocation | Mailbox permanently over quota (some servers use for hard) | Treat as hard bounce after 3+ occurrences |
| 553 | Mailbox name not allowed | Invalid syntax in email address | Fix typo or remove from list |
| 554 | Transaction failed / rejected | Server-level policy block; often blacklist-related | Remove address; check your blacklist status |
Soft bounces
Soft bounces are temporary delivery failures. The address is valid but delivery failed for a transient reason. Most ESPs retry automatically for 24–72 hours. After 3–5 consecutive soft bounces across separate sending cycles, suppress the address.
Common soft bounce SMTP codes
| SMTP Code ↕ | Name ↕ | Cause ↕ | Retry Strategy ↕ |
|---|---|---|---|
| 421 | Service temporarily unavailable | Server overloaded or temporarily down | Retry after 15–30 minutes |
| 450 | Mailbox temporarily unavailable | Greylisting or temporary mailbox issue | Retry after 1 hour |
| 451 | Local error in processing | Greylisting — first delivery attempt rejected | Retry after 5–30 minutes (automatic) |
| 452 | Insufficient system storage | Recipient inbox temporarily full | Retry for 24–72 hours; suppress if persistent |
| 455 | Server unable to process parameters | Temporary issue with headers or message content | Retry with delay; investigate content |
Bounce rate benchmarks and impact
High bounce rates signal to mailbox providers that you're sending to invalid or purchased addresses — one of the strongest spam indicators. Sustained high bounce rates damage sender reputation, reduce inbox placement, and trigger throttling or blocking.
- < 0.5%
- Hard bounce target
- < 3%
- Soft bounce target
- Immediate
- Suppression after hard bounce
- 3–5 hits
- Soft bounce suppression
Gmail/Yahoo flag senders above 2%
Investigate if consistently higher
1 strike — never retry a hard bounce
Treat as hard after repeated failures
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