Definition: hard bounce

An undeliverable email message. It is returned to the sender as "host unknown" or "recipient unknown," due to an invalid email address or possibly an expired address. A hard bounce may also occur if the receiving mail server has been configured to block all mail from the sender's domain. For example, an ISP may block a known spammer. Hard bounces are permanent for that address unless there is a deliberate change, and the address is later restored as valid.

The Soft Bounce
Contrast with "soft bounce," which is mail that is returned because the recipient's mailbox may be full, the attachment size is too large or due to some connection problem. A soft bounce is often temporary.