Definition
Error handling in email marketing refers to the systems and processes for managing delivery failures gracefully. When an email cannot be delivered, the receiving server sends a bounce message with a status code indicating the reason. Proper error handling automatically categorises, logs, and acts on these failures to maintain list quality and sender reputation.
Types of Delivery Errors
| Error Type | Status Code | Description | Action |
|---|---|---|---|
| Hard bounce | 5xx | Permanent failure (invalid address) | Remove from list immediately |
| Soft bounce | 4xx | Temporary failure (mailbox full) | Retry with back-off, remove after N failures |
| Timeout | N/A | Server did not respond | Retry with increasing intervals |
| Rejection | 5xx | Server refused the message | Investigate reason (content, reputation) |
| Deferral | 4xx | Server temporarily unavailable | Retry according to ESP schedule |
Error Handling Best Practices
- Automated bounce processing: Remove hard bounces immediately
- Soft bounce limits: Remove after 3-5 consecutive soft bounces
- Retry schedules: Exponential back-off (5min, 15min, 1hr, 4hr, 24hr)
- Category-Based handling: Different rules for different error types
- Logging and monitoring: Track error patterns by domain, campaign, and segment
- Alerting: Notify the team when error rates exceed thresholds
Common SMTP Error Codes
| Code | Meaning | Handling |
|---|---|---|
| 450 | Mailbox unavailable (temporary) | Retry with back-off |
| 550 | Mailbox not found | Hard bounce — remove immediately |
| 551 | User not local | Hard bounce — remove immediately |
| 552 | Mailbox full | Soft bounce — retry, track count |
| 553 | Mailbox name invalid | Hard bounce — remove immediately |
| 554 | Transaction failed | Investigate — may indicate content filtering |
Error Rate Benchmarks
- Hard bounce rate: Under 2% (ideal), 2-5% (needs attention), 5%+ (critical)
- Soft bounce rate: Under 5% (normal), 5-10% (monitor), 10%+ (investigate)
- Overall delivery failure rate: Under 5% (healthy)
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Frequently Asked Questions
Synchronous bounces occur during the SMTP conversation and are reported immediately to the sending server. Asynchronous bounces (also called feedback loop bounces) are reported after the message is accepted, typically via a bounce message sent to the return-path address. Both must be handled properly.
Set a threshold of 3-5 consecutive soft bounces over a rolling 7-day window before removal. This prevents premature removal for transient issues (like a full mailbox) while protecting list quality from persistently problematic addresses.
Yes. An email can be accepted by the receiving server (no bounce) but filtered to spam, blocked by a corporate firewall, or silently dropped. This is why inbox placement testing is important — error handling only catches explicitly bounced emails.