Also known as: Permanent Bounce
A permanent delivery failure that happens when an email is sent to an address that does not exist or cannot receive mail.
A hard bounce is a permanent failure. It means the receiving server rejected the message outright — usually because the mailbox does not exist, the domain is invalid, or the server is blocking you. Unlike a soft bounce, retrying will not help; the address is simply unreachable.
Hard bounces are the single biggest reason to verify a list before sending. Mailbox providers like Gmail and Outlook treat a high bounce rate as a strong signal that you are mailing a poorly maintained list, and they will start routing your mail to spam — or rejecting it — even for your valid subscribers.
The fix is prevention: validate addresses at the point of capture and clean existing lists before each campaign so dead addresses never get mailed in the first place.
A temporary delivery failure caused by a transient issue, such as a full mailbox or a server that is briefly unavailable.
The percentage of sent emails that could not be delivered and were returned by the receiving server.
The process of checking whether an email address is real, correctly formatted, and able to receive mail before you send to it.
A trust score that mailbox providers assign to your sending IP and domain, which determines whether your mail reaches the inbox.
See it in action
Check any address against BounceShift's multi-layer engine — syntax, MX, disposable, and role checks free, with full SMTP verification on signup.