What is a hard bounce? (Email glossary definition)

Hard bounce is a permanent email delivery failure. The receiving server has rejected the message with a 5xx status code, and retrying will not change the outcome. The address is gone and will not come back.

Common causes:

• The mailbox does not exist (5.1.1).
• The domain has no MX records (cannot receive mail at all).
• The mailbox was permanently deactivated.
• The user no longer exists at this organization.
• The address is malformed.

Action: remove the address from your list immediately. Every reputable ESP auto-suppresses hard-bounced addresses after the first occurrence. Continuing to send to addresses that have hard-bounced is the fastest way to wreck your sender reputation.

Pre-send verification catches most hard-bounce candidates (they return as invalid, disabled, etc.) so you can remove them before they hit your ESP and count against your bounce rate. See how to handle hard vs soft bounces.