A hard bounce is a permanent delivery failure - the mailbox does not exist and never will accept your mail. This guide explains what triggers a hard bounce, how it differs from a soft bounce, why it damages sender reputation, and how to prevent it before you hit send.
A hard bounce is a permanent email delivery failure. The receiving server returns a 5xx code - usually because the mailbox does not exist or the domain is invalid - and the message will never be delivered on retry. Suppress the address immediately.
Hard (5xx)
Soft (4xx)
For the full picture of every bounce type and SMTP code, see what is an email bounce and the bounce types guide.
Mailbox providers read your hard bounce rate as a direct signal of list quality. A high rate tells Gmail, Outlook, and Yahoo that you are mailing addresses you have not cleaned - the classic fingerprint of spam. The result is throttling, spam-foldering, and in severe cases blocking, which affects every recipient, not just the bad addresses.
Keep hard bounces under roughly 2% per send. Above that, most senders see deliverability drop sharply.
Hard bounces are almost entirely preventable, because the addresses that cause them can be identified before you send. Verify your list, remove the invalid addresses, and the hard-bounce rate collapses. Re-verify before each major send, since around a fifth of valid addresses go stale within 90 days.
Clean a list now with bulk verification, or read how to reduce bounce rate.
The 5xx or 4xx code decides the action. Treating a hard bounce as temporary, or a soft bounce as permanent, either burns reputation or deletes a live contact.
| Result | Action | Retry policy |
|---|---|---|
| Hard bounce (5xx) | Remove and suppress everywhere: ESP, CRM and transactional stream | Never retry |
| Soft bounce (4xx) | Keep the contact, pause the sequence | Retry after 24 to 72 hours, maximum 3 attempts |
| Three soft bounces in a row | Treat as hard and remove | No further retries |
| Unknown or blocked probe | Hold out of cold volume and re-check before deciding | Re-check after 7 days |
Thresholds: keep the overall bounce rate under 2%, the working limit most sending platforms enforce. A new sending domain has almost no reputation buffer, so the safe operating target for hard bounces is under 0.5% before the domain has warmed up.
Across the BounceZero corpus, 14.5% of non-freemail addresses were invalid and 27.0% could not be confirmed, which is why removal policy starts with verification, not with the bounce report. Compare the full handling rules in hard bounce vs soft bounce, or estimate the impact with the bounce rate calculator.
A hard bounce is a permanent delivery failure. The receiving server refuses the message with a 5xx code - usually because the mailbox does not exist or the domain is invalid - and it will never be delivered no matter how many times you retry. The address should be removed from your list at once.
A hard bounce is permanent (5xx): the mailbox or domain does not exist, so retrying is pointless. A soft bounce is temporary (4xx): a full mailbox, a server that is down, or greylisting - it may succeed on a later attempt. Hard bounces must be suppressed immediately; soft bounces are retried a few times before suppression.
The main causes are a mailbox that does not exist (a typo, or a person who left the company), an invalid or expired domain, and a server that permanently blocks the sender. The common thread is that the address is simply not deliverable.
Verify addresses before you send. Running a list through an email verifier removes the invalid mailboxes that cause hard bounces, so your bounce rate stays under the ~2% that mailbox providers tolerate. Re-verify before major sends, since lists decay over time.
Verify the list, drop the dead addresses, keep bounce rate under 2%. 100 free verifications.
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Hard vs soft bounces, SMTP codes, and how to fix them
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