Hard bounces are email addresses that have permanently failed. This means that the email address no longer exists, is invalid, or is unable to receive email. Hard bounces can occur for a variety of reasons, but the most common cause is an invalid email address. When an email is sent to an invalid address, it will immediately bounce back to the sender. Hard bounces can also occur if an email server is down or if the recipient’s inbox is full. Hard bounces can damage your sender reputation and should be removed from your list as soon as possible. To avoid hard bounces, make sure to use a valid email list and to regularly clean your list of inactive or invalid addresses.