Definition
Email bounce codes are numeric codes returned by receiving mail servers to explain why an email message was not delivered. These codes are defined by the SMTP protocol and categorise delivery failures into temporary (4xx) and permanent (5xx) status.
Understanding bounce codes is essential for email marketers. They tell you exactly why an email bounced — whether the address is invalid, the mailbox is full, the server rejected the message, or a policy blocked delivery — and what action you should take.
Common SMTP Bounce Codes
| Code | Meaning | Action Required |
|---|---|---|
| 421 | Service temporarily unavailable | Retry later |
| 450 | Mailbox unavailable | Retry later |
| 451 | Local processing error | Retry later |
| 452 | Insufficient system storage | Retry later |
| 500 | Syntax error | Check email format |
| 501 | Syntax error in parameters | Check email address format |
| 550 | Mailbox not found | Remove immediately (hard bounce) |
| 551 | User not local | Check email domain |
| 552 | Mailbox full | Retry, then remove after repeated attempts |
| 553 | Mailbox name not allowed | Remove immediately (hard bounce) |
| 554 | Transaction failed | Check policies (may indicate spam rejection) |
4xx vs 5xx Bounce Codes
| Type | Code Range | Meaning | Sender Action |
|---|---|---|---|
| Temporary (soft bounce) | 4xx | Temporary delivery failure — mailbox full, server busy | Retry automatically (typically 3-5 attempts over 72 hours) |
| Permanent (hard bounce) | 5xx | Permanent delivery failure — address invalid, domain does not exist | Remove immediately, do not retry |
Bounce Codes by Category
| Category | Common Codes | Example Scenario |
|---|---|---|
| Invalid address | 550, 553 | Email address does not exist |
| Mailbox full | 552 | Recipient has exceeded storage limit |
| Server error | 421, 451 | Receiving server is temporarily down |
| Rate limiting | 452 | Sending too fast for the receiving server |
| Content rejection | 554 | Message flagged as spam or policy violation |
| DNS failure | 450 | Recipient domain cannot be resolved |
| Authentication failure | 550 5.7.1 | SPF, DKIM, or DMARC check failed |
How to Handle Bounce Codes in Email Marketing
- Hard bounces (5xx): Remove immediately from your active list. Retrying hard bounces damages sender reputation and increases bounce rates.
- Soft bounces (4xx): Retry automatically 3-5 times over 48-72 hours. If the soft bounce persists, suppress the address. Repeated soft bounces may indicate a permanent problem.
- Policy rejections (554): Review your authentication configuration and sending practices. Policy rejections often indicate DMARC failures, blacklisting, or content flagged as spam.
- Temporary server errors (421, 451): These are transient and usually resolve on retry. Do not suppress addresses for temporary errors.
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Frequently Asked Questions
A 550 code means the specific mailbox does not exist on the receiving server. The address is invalid and should be removed. A 554 code means the message was rejected for policy reasons — this could be spam detection, DMARC failure, or IP blacklisting. 554 requires you to review your sending practices, not just remove the address.
No. 5xx errors are permanent failures. Retrying them wastes resources and signals poor list hygiene to receiving servers. Remove 5xx bounces from your active list immediately.
Most ESPs provide bounce code details in their delivery logs or activity reports. If your ESP does not expose raw bounce codes, request this data from support or consider switching to a platform that provides granular delivery data.
Spam traps typically return a 550 code (mailbox not found) because the address is valid in format but does not belong to a real person. You cannot distinguish a spam trap from a regular invalid address by bounce code alone — that is why list hygiene that removes all invalid addresses is essential.
Some receiving servers return generic bounce messages without specific codes, especially when using custom rejection pages. These are called "unknown bounces" and should be treated as soft bounces — retry a limited number of times, then suppress if the condition persists.