What are email bounces?
An email bounce occurs when a receiving mail server rejects a message and returns it to the sender with an error. The sending system receives an SMTP response code that indicates why the delivery failed. Proper bounce handling is essential for maintaining good sender reputation and deliverability.
Hard vs Soft Bounces
The two main categories:
- Hard bounce: Permanent failure. The address is invalid, the domain does not exist, or the recipient is blocked.Action: Immediately suppress the address.
- Soft bounce: Temporary failure. Examples include a full mailbox, temporary server issue, or rate limiting.Action: Retry with backoff, typically up to 72 hours.
Common SMTP Bounce Codes
| Code | Type | Meaning | Recommended Action |
|---|---|---|---|
| 421 | Soft | Service temporarily unavailable | Retry with backoff |
| 450 | Soft | Mailbox temporarily unavailable | Retry |
| 452 | Soft | Insufficient storage | Retry |
| 4.7.28 | Soft | Reputation / rate limit (Gmail) | Slow down, check reputation |
| 550 | Hard | Address does not exist | Suppress permanently |
| 551 | Hard | User not local | Suppress |
| 553 | Hard | Invalid mailbox name | Suppress |
| 5.7.1 | Hard | Policy violation | Check authentication & content |
| 5.7.26 | Hard | Authentication failure | Fix SPF/DKIM/DMARC |
Common Causes of Bounces
- Invalid or outdated email addresses (typos, closed accounts)
- Mailbox full or over quota
- Authentication issues (SPF, DKIM, or DMARC failures)
- Sending IP or domain on a blocklist
- Low sender reputation due to complaints or rapid volume increases
- Content triggers (phishing-like patterns)
- Recipient-side forwarding disabled (e.g., Apple Private Relay)
Bounce Remediation Playbook
- Always examine the full bounce reason and SMTP code.
- Immediately suppress all hard bounces — never send to them again.
- Retry soft bounces using exponential backoff (stop after 4–6 attempts).
- Look for patterns: A spike in one code often points to a specific issue (authentication, rate limiting, etc.).
- Periodically re-validate older addresses before re-engaging them.
- Monitor sender reputation tools (Postmaster Tools, DMARC reports, etc.).
Automating Bounce Processing
Connect your sending provider’s bounce notifications (via webhooks or logs) to your CRM or customer database. Automatically set a do_not_email flag on hard bounces and repeated soft bounces. Set up alerts if your overall bounce rate exceeds safe thresholds.
Bounce Rate Thresholds
- Healthy: Under 2%
- Warning: 2–5% — investigate promptly
- Critical: Over 5% — major deliverability risk
High bounce rates can damage your domain and IP reputation. Combine good bounce handling with proper authentication and gradual domain warming.