Unknown User
Bounced - Unknown User is an error indicating the email address does not exist.
If an email address (user) does not exist at the organization or domain, it will receive Unknown User response (a hard bounce error).
An Unknown User response is typically due to SMTP 5.5.0 errors which cause an email to be undeliverable to an inactive, incorrect, or disabled recipient address.
### Additional Resources * Delivery Metrics * Bounces * Subscriber List Management
