// Glossary
Hard Bounce
A permanent delivery failure, usually because the address doesn't exist or the domain can't receive mail.
Definition
A hard bounce is a permanent email delivery failure caused by an issue that won't resolve on retry, such as a nonexistent mailbox, an invalid or dead domain, or a server that blocks the sender outright. Mailbox providers treat repeated hard bounces as a sign of a poorly maintained list, which damages sender reputation. Verifying addresses before sending is the main way to prevent hard bounces.