A hard bounce is a permanent reason why an email cannot be delivered to a recipient. It occurs in the following situations:

• A sender incorrectly types a user’s email address.
• A recipient's email address does not exist on the domain or email server.
• The recipient’s domain does not exist at all (xyz@domain.com).

All of these situations are permanent because no matter how many times the sender tries to resend the email message, the sender will continue to receive a hard bounce email. Once the email is rejected by the recipient's email server (when the domain exists, but the recipient does not), an auto-generated bounced email is sent by the recipient's domain server specifying that the user does not exist on that domain. Similarly, when a recipient domain is invalid, the sender’s host domain will auto-generate a bounced email indicating that the specified domain does not exist or cannot be reached.

A hard bounce also occurs when an email message has been rejected by the recipient's email server for privacy or security reasons (email/email domain/recipient tagged as spam/spammer).