Glossary · Email
Hard bounce
A permanent delivery failure: the address does not exist and never will.
A hard bounce is the receiving server saying the recipient is invalid, not that it is busy or full. The message cannot be delivered by retrying. Mailbox providers watch the rate of hard bounces closely because a high rate is the signature of a purchased or scraped list.
Hard bounces are what pre-send verification exists to prevent. Every one that reaches a provider is a data point that your sending domain is not careful about who it mails.
Once an address hard bounces it should be suppressed permanently. Re-sending to a known bad address is one of the fastest ways to lose sender reputation.