What Is an Email Bounce?
An email bounce happens when your email fails to reach a recipient's inbox. Instead of being delivered, it's returned (or "bounced") by the receiving mail server.
There are two main types:
- Hard bounce: Permanent failure
- Soft bounce: Temporary failure
Both affect your sender reputation—but in different ways. Managing bounces is crucial for anyone sending email at scale, especially for SaaS teams, marketers, or sales-led startups.
Hard Bounce vs. Soft Bounce: The Core Differences
| Bounce Type | Definition | Recovery Possible? |
|---|---|---|
| Hard Bounce | Permanent delivery failure due to invalid address or domain | ❌ No |
| Soft Bounce | Temporary issue like full inbox, timeout, or message size | ✅ Yes (retry later) |
Hard Bounce Example:
Sending to jane@wrongdomain.xyz returns a "550 No Such User Here" error.
Soft Bounce Example:
Sending to mark@company.com fails due to a full inbox, but may succeed next time.
Causes of a Hard Bounce
Hard bounces typically result from:
- Invalid or mistyped email addresses
- Non-existent domains (e.g., @gmial.com)
- Deactivated or deleted inboxes
- Blocked senders (your domain or IP is blacklisted)
They're signals that your list may be outdated, purchased, or poorly maintained.
Causes of a Soft Bounce
Soft bounces usually stem from short-term issues:
- Recipient inbox is full
- Mail server is down or slow
- Message is too large (oversized attachments)
- Spam filtering temporarily delayed delivery
Soft bounces often resolve on retry—but repeated soft bounces can eventually turn into hard bounces.
How Bounce Rates Affect Email Deliverability
Inbox providers like Gmail and Outlook monitor bounce rates closely. Too many bounces can lead to:
- Lower inbox placement
- IP/domain reputation damage
- Email throttling or outright blocking
Industry benchmarks:
- ✅ Bounce rate under 2% = healthy
- ⚠️ 2–5% = risk zone
- ❌ 5%+ = serious deliverability issues
For a deeper dive, check out Email Deliverability 101 ← (internal link to P2)
Diagnosing Bounces: What to Look for in Reports
Modern email tools provide bounce classifications via:
- SMTP response codes (e.g., 550, 421, 552)
- ESP dashboards (like SendGrid, Postmark, Brevo)
- Bounce reason logs (e.g., invalid recipient, greylisting, spam rejection)
Understanding these codes helps troubleshoot and segment risky emails from your main sends.
How to Prevent Hard Bounces
The best way to avoid hard bounces? Never send to an invalid address.
Actionable tips:
- Use an email validation API to check every address before it enters your list
- Avoid buying email lists
- Set up double opt-ins for high-value contacts
- Clean your list quarterly, at minimum
We explain these tactics in detail in Email Validation: The Definitive Guide ← (internal link to P1)
How to Minimize Soft Bounces
You can't eliminate all soft bounces, but you can reduce them:
- Retry undelivered messages over 48–72 hours
- Compress or remove large attachments
- Avoid image-heavy content with poor formatting
- Throttle volume for cold campaigns—don't send 5,000 emails on day one
Many ESPs will handle retries for you—but poorly formatted or aggressive campaigns can lead to bounces that never recover.
Role of Email Validation in Bounce Prevention
Email validation tools like Lero prevent hard bounces by:
- Catching typos (gmal.com, outllok.com)
- Blocking disposable or expired inboxes
- Flagging catch-all domains or spam traps
- Running SMTP checks in real time
💡 Bonus:
Lero gives you 500 free checks, responds in <50ms, and costs just $0.0003 per check. Perfect for startups and cold outreach.
Want to know more? Explore the complete email validation guide ← (internal link to P1)
Improving Deliverability by Managing Bounce Rates
Managing bounce rates is just one piece of the email deliverability puzzle.
Other key practices include:
- SPF/DKIM/DMARC setup
- Warming up domains and IPs
- Segmenting lists by engagement
- Monitoring inbox placement tools (like GlockApps)
If you're serious about inbox placement, read Email Deliverability 101 ← (internal link to P2)
Bounce Prevention Tools to Know
| Tool | Use Case |
|---|---|
| Lero | Real-time + bulk email validation |
| Mail-Tester | Test deliverability before campaigns |
| SendForensics | Monitor domain reputation & inbox rate |
| Postmark/Mailgun | Bounce analytics from ESP logs |
FAQs About Email Bounces
Q1: What's an acceptable bounce rate?
Below 2% is ideal. Anything above 5% could harm your reputation.
Q2: Can soft bounces become hard bounces?
Yes. If an inbox remains full or a server issue persists, ESPs may eventually treat it as a hard bounce.
Q3: Will my ESP automatically stop sending to hard-bounced addresses?
Most reputable ESPs will suppress them—but you should clean them manually too.
Q4: How often should I clean my list?
At least every 3 months—or before major campaigns.
Q5: What does "550 User Unknown" mean?
That's a hard bounce. The recipient's email address doesn't exist.
Q6: Is real-time validation better than bulk cleaning?
Both matter. Use real-time to stop bad data from entering. Use bulk to clean legacy lists.
Final Thoughts & Next Steps
Understanding the difference between hard and soft bounces is crucial for:
- Sending smarter emails
- Maintaining deliverability
- Protecting your brand reputation
📚 Want to go deeper?
- • Read the Email Validation Guide (P1)
- • Or explore Deliverability 101 (P2)