What is a hard bounce and how does it affect sender reputation?

• A. An email returned permanently; signals to ISPs that a mailbox doesn't exist or is invalid, harming reputation if frequent.
• B. A temporary delivery delay.
• C. A message delivered but not opened.
• D. A spam filter grade.

Answer: (A)

A hard bounce is a permanent delivery failure: the email can’t be delivered to the recipient’s mailbox for reasons such as the address or domain not existing. Because this is a permanent issue, ISPs interpret it as a sign that the sender’s list contains invalid addresses. When hard bounces happen frequently, they harm sender reputation, making ISPs more likely to throttle or block future messages and hurt overall deliverability. This is in contrast to soft bounces, which are temporary problems like a full mailbox or server hiccups. To protect reputation, remove hard-bounced addresses promptly, keep your list clean, consider double opt-in, verify addresses before sending, and monitor bounce rates regularly.