Define "hard bounce" in email marketing.

A "hard bounce" refers to an email that cannot be delivered due to a permanent issue with the recipient's email address. This could occur for several reasons, such as the email address being invalid, the domain not existing, or the email server rejecting the message due to policy or configuration settings. When an email hard bounces, it indicates that the problem is not temporary; therefore, the sender should remove this email address from their mailing list to maintain list hygiene and ensure effective email marketing practices.

Temporary undeliverables, indicated by a "soft bounce," differ in nature since they might be caused by a full mailbox or server issues, which could resolve over time. Other options like unsubscribing after an email is opened or marking an email as spam do not fit the definition of a hard bounce since they point towards user actions rather than issues with the email address itself. Understanding these distinctions is crucial in managing email campaigns and maintaining an effective email marketing strategy.