Glossary
Hard bounce
Last reviewed 18 September 2026
What is a hard bounce?
A hard bounce is a permanent refusal of a message by the receiving server, carried in a 5xx SMTP response, because the mailbox does not exist, the domain does not exist, or the address has been disabled.
It must be suppressed immediately and permanently, on the first occurrence, not after three and not after a review. Continuing to send to an address you have been told does not exist is the clearest signal available to a receiving provider that a sender does not process its bounces, and not processing bounces is the operational signature of a bought or scraped list.
It is also how you keep mailing a spam trap, which is an address that exists only to be mailed by people who should not have it. A confirmation step removes most of the addresses that would hard bounce before they are ever sent to a second time.
See also
- Soft bounce
A soft bounce is a temporary refusal of a message by the receiving server, carried in a 4xx SMTP response, because the mailbox is full, the server is greylisting you, or you are being rate-limited.
- Suppression list
A suppression list is the permanent record of addresses that must never be mailed again, holding unsubscribes, complaints and hard bounces, and it is checked before every send rather than after one.
- List hygiene
List hygiene is the work of keeping a list to addresses that exist and belong to people who still want the mail: hard bounces suppressed on the first failure, long-silent addresses retired, typos caught at the form, and nothing re-imported that was already suppressed.
- Delivery rate
A delivery rate is the share of a send that the receiving mail server accepted instead of bouncing, and it says nothing at all about whether any of it reached an inbox.
Read next
This term is covered properly in Email deliverability for small senders: authentication and reputation, which is a longer piece on the subject.
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