Hard- and soft bounces are differentiated as having permanent and temporary delivery problems, respectively.

If the recipient’s email address does not exist (hard bounce), they will be blocked in the address book and allocated a stop sign. If the email address is temporarily unavailable (e.g. inbox full), the email cannot be delivered, but the recipient will only be blocked after 2 further subsequent unsuccessful delivery attempts and allocated a stop sign.

For you this means that some recipients may not have received one or more newsletters due to a soft bounce, but they remain active in your address book and can receive newsletters.