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 tell providers the list is stale or scraped, and sending platforms act on them: Amazon SES asks for bounces below 2 percent and reviews accounts at 5.
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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
Mailbox providers treat bounces as a sign of poor list quality but don't publish a limit. Sending platforms do: Amazon SES may pause an account at a 10 percent bounce rate, and many cold-outbound tools pause sending on their own when bounces 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, because people change jobs and their old addresses stop working
- Avoid scraped lists, they have much higher hard-bounce rates than verified-source lists
Track real engagement, not Apple MPP noise.
Start Free TrialFrequently 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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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.