In the realm of email marketing, a hard bounce refers to an email that is returned to the sender because it is permanently undeliverable. This could be due to a non-existent email address, a domain that no longer exists, or a recipient email server blocking your emails. When a recipient unsubscribes from your email list, their email address is usually set to hard bounce to ensure that they no longer receive communications from you. This is known as an unsubscribe hard bounce.