This setting determines how long someone who has asked to subscribe to the mailing list, but never "confirmed" their membership should remain in the system until they are permanently
erased from the system. By default, un-confirmed members are never purged. You can specify any number of days (from 1 day to 1 year) before they are erased.
This setting only applies if the mailing list is set to require confirmation from new members. See Confirm Subscribes.