Hard Bounce vs Soft Bounce: What's the Difference and What to Do
Hard bounces and soft bounces require different responses. Learn the difference, what causes each type, and exactly how to handle them to protect your sender reputation.
Not all email bounces are created equal. A hard bounce and a soft bounce mean very different things — and they require very different actions. Getting this wrong can either damage your sender reputation (ignoring hard bounces) or shrink your list unnecessarily (removing soft bounces too aggressively).
Quick Definition
| Hard Bounce | Soft Bounce | |
|---|---|---|
| Type | Permanent failure | Temporary failure |
| SMTP code | 5xx (550, 551, 553, 554) | 4xx (421, 450, 451, 452) |
| Meaning | Will never work | Might work later |
| Action | Remove immediately | Retry, then remove if persistent |
| Reputation impact | Severe — signals list quality issues | Moderate — acceptable in small numbers |
Hard Bounces: Permanent Failures
A hard bounce means the email will never be delivered to this address. The receiving server has permanently rejected it.
Common causes
- 550 5.1.1 — User unknown: The mailbox doesn't exist. Most common hard bounce.
- 550 5.1.2 — Domain not found: The domain doesn't exist or has no MX records.
- 553 — Mailbox name not allowed: Invalid mailbox format on the server.
- 550 5.7.1 — Blocked by policy: Server permanently rejects your sending domain/IP.
What to do with hard bounces
- Remove from your list immediately. Never send to this address again.
- Add to a suppression list. Prevent it from being re-imported.
- Check for typos.
user@gmial.comis a hard bounce but the intended addressuser@gmail.commight be valid. - If bounces are widespread, your list needs bulk verification.
Soft Bounces: Temporary Failures
A soft bounce means the email couldn't be delivered right now, but might succeed later.
Common causes
- 450 — Mailbox busy / Greylisting: Server is temporarily rejecting as anti-spam. See our SMTP codes guide for details on greylisting.
- 452 — Mailbox full: Recipient's inbox is at capacity.
- 421 — Service unavailable: Server is overloaded or rate-limiting you.
- 451 — Local processing error: Temporary server-side issue.
What to do with soft bounces
- Let your MTA retry automatically. Most mail servers retry 3-5 times over 24-72 hours.
- Track repeat offenders. If an address soft-bounces 3+ times across different sends, treat it as a hard bounce.
- Monitor for patterns. Many soft bounces from one domain? That domain may be rate-limiting you.
The Dangerous Gray Area
Some bounces don't clearly fit either category:
- 552 — Storage exceeded: Technically a 5xx code (permanent), but the mailbox exists — it's just full. Treat as soft bounce initially, remove after 2-3 occurrences.
- 550 with "temporarily" in the message: Some servers use 550 for temporary blocks (wrong SMTP semantics). Check the message text, not just the code.
- Deferred (no code): Your MTA queued the message for retry. This is a soft bounce in progress.
Bounce Rate Benchmarks
| Bounce Rate | Assessment |
|---|---|
| Hard bounce under 0.5% | Excellent — clean, verified list |
| Hard bounce 0.5-2% | Acceptable — normal list decay |
| Hard bounce over 2% | Dangerous — verify your list before next send |
| Soft bounce under 5% | Normal — server-side temporary issues |
How Verification Prevents Bounces
Email verification catches most hard bounces before they happen by checking addresses pre-send:
- Invalid mailbox (550) — Caught by SMTP verification
- Invalid domain — Caught by DNS check
- Invalid syntax — Caught by syntax validation
- Disposable addresses — Caught by blocklist check
Verification can't prevent all soft bounces (they're server-side temporary issues), but it eliminates the addresses that would hard bounce.
Prevent bounces before they happen
Sign up free for 100 verification credits or try our free checker to test individual addresses. See our complete guide to reducing bounce rate for more strategies.
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