Hard Bounce
A hard bounce is a permanent delivery failure: the receiving server rejects the message with a 5xx SMTP code and the sending platform should not retry. Common causes are invalid mailbox address, non-existent domain, or sender domain blocked. Hard bounces are the single most damaging signal to sender reputation; a hard-bounce rate above 2 percent will degrade inbox placement materially within days.
5xx SMTP responses are hard bounces. 550 (mailbox unavailable, permanent), 551 (user not local), 553 (mailbox name invalid), 554 (transaction failed) are the common codes. The receiver is saying no, do not try again.
Why hard-bounce rate matters
Gmail, Microsoft, and Yahoo measure hard-bounce rate as a signal that the sender's list quality is poor. Sustained rates above 2 percent move the sender into spam folder by default within days. Cold-outbound platforms increasingly enforce hard-bounce limits and pause sending automatically when rates spike.
How to keep hard-bounce rate low
- Use a verification service (NeverBounce, ZeroBounce, Hunter Verifier) on every list before importing
- Suppress hard-bounced addresses immediately and permanently
- Re-verify lists older than 90 days. up to 2 percent of B2B addresses go invalid per quarter as people change jobs
- Avoid scraped lists. they have much higher hard-bounce rates than verified-source lists
Frequently asked questions
Is 2 percent the universal limit?+
It is a widely accepted rule of thumb. Google's bulk-sender rules effective February 2024 require sustained complaint rate under 0.3 percent and emphasise list quality; sustained hard-bounce rate above 2 percent typically correlates with that limit being breached.
What about catch-all domains?+
Catch-all domains accept any `anything@domain` and return a soft bounce or silent ignore rather than a hard bounce. Hard-bounce verification cannot rule them out. Catch-alls should be flagged as risky and sent low-volume.
Put this concept into practice
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Related glossary terms
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We update these pages when the underlying mechanics change. new mailbox-provider rules, new tracker behavior, new measurement gaps. The dates above are real revisions, not auto-touches.