Hard Bounce
A permanent delivery failure typically caused by invalid email addresses.
1 min readLast updated Apr 2026
Quick Reference
CategoryEmail Marketing & Klaviyo
Related Terms2
A permanent delivery failure typically caused by invalid email addresses.
Why It Matters
Hard bounces are dead ends that actively damage your sender reputation. ISPs track how many hard bounces you generate—high rates signal you're sending to unverified lists, which can get your domain blacklisted. Immediate removal is critical.
Practical Example
Scenario
A camping gear brand sees 250 hard bounces from a 10,000-email campaign.
Calculation
250 ÷ 10,000 × 100 = 2.5% hard bounce rateResult
At 2.5% hard bounces, they're in danger zone. They trace the issue to a pop-up form with no validation—people entering fake emails for discounts. Adding email validation reduced hard bounces to 0.3%.
Pro Tips
- 1Configure your ESP to auto-suppress hard bounces after first occurrence
- 2Add real-time email validation to signup forms to catch invalid addresses immediately
- 3Audit new subscriber sources monthly to identify which generate the most hard bounces
- 4Use domain-level validation to catch typos like 'gmial.com' before they become bounces
Common Mistakes to Avoid
Manually reviewing hard bounces instead of auto-suppressing immediately
Re-importing old lists that contain previously hard-bounced addresses
Not validating emails at point of collection, allowing fake/mistyped addresses in