Hard bounce rate refers to the percentage of emails that are not delivered to the intended recipient due to permanent and unrecoverable reasons, such as an invalid or non-existent email address. This can occur when the email address is no longer in use, the domain name is incorrect, or the recipient’s server has blocked the email. Unlike soft bounces, which are temporary delivery failures, hard bounces indicate a permanent failure and can negatively impact email marketing campaigns by reducing the overall deliverability and success rate.