Hard bounce
A hard bounce is a permanent delivery failure, usually because the address does not exist or the domain cannot receive mail.
How it works
It differs from a soft bounce, a temporary failure such as a full mailbox or a busy server that may succeed on retry. Mail servers signal hard bounces with permanent error codes in the 500 range.
Mailbox providers track how often you send to dead addresses. A high hard-bounce rate harms your sender reputation, so remove hard-bounced addresses at once and validate lists before large sends.
Related LookValid products
| Product | Credits | Docs |
|---|---|---|
| Email Validation (mailbox verification) | 1 credit per check | Product page |
| Email Domain Check | 1 credit per address checked | API docs |
New to the API? Start with the quickstart or the API reference. Agents can call the same lookups through the MCP server.