An email bounce occurs when a message is returned to the sender because it couldn't be delivered. Hard bounces are permanent failures — the address doesn't exist, the domain is invalid, or the server permanently rejects the message. Soft bounces are temporary — the mailbox is full, the server is down, or the message is too large. High bounce rates damage sender reputation and can trigger spam filtering.
What Is a Bounce in Email?
Bounces: When Email Can't Be Delivered
A bounce means the email you sent was returned — the receiving server couldn't or wouldn't deliver it. Every bounce comes with an SMTP response code explaining why.
Hard Bounces (5xx)
Permanent failures. The email will never be deliverable to this address.
Common causes:
550 User not found— the address doesn't exist550 Domain not found— the domain has no MX records550 Mailbox disabled— the account was deactivated553 Relaying denied— server won't accept for this domain
Action: Remove hard bounces from your list immediately. Never retry them. Continuing to send to hard-bouncing addresses damages reputation.
Soft Bounces (4xx)
Temporary failures. The email might be deliverable later.
Common causes:
450 Mailbox full— recipient's inbox is at capacity451 Try again later— server is temporarily overloaded452 Too many connections— rate limiting421 Service not available— temporary server issue
Action: Your ESP will retry soft bounces automatically (typically 3-5 times over 24-72 hours). If an address soft bounces on 3+ consecutive sends, remove it.
For complete code references, see SMTP 5xx rejection codes and SMTP 4xx deferral codes.
Bounce Rate Thresholds
| Rate | Status | Action |
|---|---|---|
| Under 1% | Healthy | Maintain current practices |
| 1-2% | Caution | Clean list, check collection methods |
| 2-5% | Problem | Immediate list cleaning, pause growth |
| Over 5% | Critical | Stop sending, validate entire list |
Practitioner note: The most common cause of sudden high bounces I see is someone importing a CSV that wasn't validated. One bad import can spike your bounce rate and damage reputation that took months to build. Always validate before importing.
Practitioner note: Soft bounces at Microsoft (Outlook/Hotmail) are often reputation-based — they're telling you to slow down, not that the mailbox is actually full. If you see mass soft bounces at Microsoft, check your IP reputation via SNDS.
For bounce management best practices, read bounce handling explained. If your bounce rate suddenly increased, see the troubleshooting guide.
Struggling with high bounce rates? Schedule a consultation — I'll audit your list quality and collection practices to fix the root cause.
Sources
- RFC 5321: Simple Mail Transfer Protocol (bounce handling)
- RFC 3463: Enhanced Mail System Status Codes
- Google: Email Sender Guidelines
- ZeroBounce: Email Validation Best Practices
v1.0 · April 2026
Frequently Asked Questions
What's the difference between a hard bounce and a soft bounce?
A hard bounce is permanent — the email address doesn't exist or the domain is invalid. Remove hard bounces immediately. A soft bounce is temporary — the mailbox is full, the server is overloaded, or the message is deferred. Retry soft bounces, but remove addresses that soft bounce repeatedly.
What is a good bounce rate?
Keep bounce rate below 2% total. Hard bounce rate should be under 0.5%. Most ESPs will suspend your account if hard bounces exceed 5%. For well-maintained lists, total bounce rate should be under 1%.
How do I reduce bounce rate?
Validate email addresses at collection (real-time verification), clean your list regularly with validation tools (ZeroBounce, NeverBounce), remove hard bounces immediately, implement double opt-in, and remove addresses that soft bounce three or more times.
Do bounces affect deliverability?
Yes. High bounce rates signal to mailbox providers that you're sending to an unclean list, which damages sender reputation. Hard bounces are particularly damaging — they indicate you're sending to addresses that don't exist, a hallmark of purchased lists.
Why do I suddenly have high bounces?
Common causes: you sent to an old list that hasn't been cleaned, you imported a new list with bad addresses, a receiving server changed its configuration, or you're experiencing a temporary server issue at the receiving end.
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