What causes hard bounces?

Hard bounces occur when the recipient’s address is invalid. The domain may not exist. The mailbox may never have existed, often returning an SMTP 550 User unknown error. The domain may have expired. Or the provider may have shut the account down.

A common cause is poor data collection such as typos or purchased lists. When [email protected] is typed as [email protected], the server rejects it with a permanent failure.