Bounce
An email that could not be delivered and is returned to the sender with an error message.
Definition
An email bounce occurs when a message cannot be delivered to the recipient's email address. The receiving server rejects the email and sends a bounce message (also called a Non-Delivery Report or NDR) back to the sender explaining why delivery failed. Bounces are classified as either hard bounces (permanent failures) or soft bounces (temporary issues). Understanding bounces is essential for maintaining list quality and sender reputation.
Why It Matters
High bounce rates damage your sender reputation and can get you blocklisted by email providers. ISPs view high bounces as a sign of poor list quality or spammy behavior. Managing bounces promptly is essential for maintaining deliverability. Google requires bounce rates under 2% for bulk senders.
How It Works
When an email cannot be delivered, the receiving mail server generates a bounce message containing an error code and description. SMTP error codes starting with 5xx indicate permanent failures (hard bounces), while 4xx codes indicate temporary issues (soft bounces). Your email service provider receives this bounce and should automatically categorize it and update the subscriber's status.
Example
You send a campaign to 10,000 subscribers. The delivery report shows:
- 9,500 delivered
- 300 soft bounces (mailbox full or server busy)
- 200 hard bounces (invalid addresses)
Your bounce rate is 5% - which is too high.
Looking at the hard bounces, you see many are from a list you imported last month without verification. You remove all hard bounces immediately and verify the remaining list.
Soft bounces are retried over 48 hours - most eventually deliver. After cleanup, your next campaign has only 0.5% bounces.
Best Practices
- 1Remove hard bounces from your list immediately after the first occurrence
- 2Monitor soft bounces and remove after 3-5 consecutive failures
- 3Use email verification before sending to new or imported addresses
- 4Keep overall bounce rates below 2% to maintain good reputation
- 5Check bounce reasons to identify patterns (old list, fake signups, etc.)
Automatic Bounce Handling
Sequenzy automatically processes bounces, categorizes them, and updates subscriber statuses to protect your reputation.
Learn MoreThe ranges and timings below are illustrative planning examples, not measured industry benchmarks or universal targets. Set thresholds using your own audience, product cadence and historical data.
Bounce Type Table
| Bounce type | Meaning | Action |
|---|---|---|
| Hard bounce | Permanent failure such as invalid mailbox | Suppress immediately |
| Soft bounce | Temporary issue such as mailbox full | Retry and suppress after repeated failures |
| Block bounce | Receiving server rejected due to reputation or policy | Investigate deliverability cause |
| Technical bounce | DNS, connection, or server error | Monitor and retry if temporary |
Bounce Rate Thresholds
| Bounce rate | Status | Recommended response |
|---|---|---|
| Under 1% | Healthy | Keep normal list hygiene |
| 1-2% | Watch | Review new sources and hard bounces |
| 2-5% | Risky | Clean list and reduce risky sends |
| Above 5% | Serious | Pause imports or campaigns causing bounces |
Bounce Reason Table
| Bounce reason | Likely cause | Prevention |
|---|---|---|
| Mailbox does not exist | Typo, old address, fake signup | Validate addresses and confirm opt-in |
| Domain does not exist | Bad import or malformed address | Clean imports before sending |
| Mailbox full | Abandoned or overloaded inbox | Suppress after repeated soft bounces |
| Message blocked | Reputation, content, or authentication issue | Check auth, blocklists, and complaints |