Definition
A hard bounce is a delivery failure the sender classifies as permanent. SMTP 5xx responses often indicate permanent failure, but the enhanced status code and diagnostic text are more useful than the label alone.
An address that does not exist should usually be suppressed immediately. Some permanent policy rejections can be specific to the message, domain, or sender rather than proof that the recipient address is invalid.
Why it matters
Repeatedly sending to known-invalid recipients wastes capacity and signals weak audience controls. Correct classification protects reputation while avoiding the accidental suppression of valid recipients after ambiguous failures.
How it works
- 01
The receiving server rejects the recipient or message with a permanent response.
- 02
The sending provider parses the SMTP and enhanced status codes.
- 03
The event is classified by cause, such as unknown user, policy, authentication, or content.
- 04
The address or route is suppressed when the cause justifies it.
Example
550 5.1.1 The email account that you tried to reach does not exist
Class: permanent
Action: suppress this address and inspect how it entered the listCommon mistakes
- Retrying a confirmed nonexistent address.
- Treating every 5xx policy response as an invalid recipient.
- Deleting bounce evidence instead of retaining classification and source context.
- Importing old lists without validating consent and recent reachability.
Key takeaways
- Suppress confirmed unknown users promptly.
- Read the enhanced status code and diagnostic.
- Track bounce source so acquisition problems can be fixed.