When you email your clients using the Client Mailing List, a custom link will be shown at the bottom to allow your users to unsubscribe from the newsletter. If you do not want this link to appear, such as in the case of a mandatory announcement, set this option to "No". For most mailings, this option should be set to "Yes", to provide your users with a choice and prevent them from complaining about Spam.

The default setting can be set in the Preferences section.