Definition:
A hard bounce is a permanent failure that prevents an email from being delivered to the recipient’s address. The receiving server rejects the message because the account, domain or delivery route is permanently invalid.
It differs from a soft bounce, which results from a temporary situation such as a full mailbox or a transient server problem. The precise classification depends on the response code and message returned by the receiving system.
Table of contents
Causes of hard bounce
A hard bounce is usually associated with an address that cannot receive messages permanently. Common causes include:
- Non-existent account: The address never existed or has been deleted by its provider.
- Address error: The mailbox name contains a typing error that prevents the recipient from being found.
- Invalid domain: The domain is misspelled, does not exist or has no working email configuration.
- Deactivated address: The account has become permanently unavailable.
- Permanent policy rejection: A server may return a permanent code because of its security or reputation rules. Not every block or classification as spam is by itself a hard bounce, so the returned response needs to be reviewed.
There is no universal acceptable percentage for every campaign. An increase in hard bounces may indicate acquisition errors, old data, poor imports or addresses entered without validation.
Consequences to consider
An address confirmed as a hard bounce should no longer receive new messages. Instead of deleting it without retaining information, platforms commonly add it to a suppression list to prevent retries and preserve a record of the failure.
Repeatedly sending to non-existent addresses wastes resources and may harm sender reputation and deliverability. It also makes campaign metrics harder to interpret by increasing the number of messages that never had an opportunity to reach a person.
Response codes should be reviewed, obvious errors corrected only when a reliable source is available, and acquisition and validation processes maintained to reduce incorrect addresses. Bounce management helps keep a list operational, but it does not replace consent, sender identification or other requirements that apply to email marketing.