The "town" of Knoxville no longer exists; it may never have ... the term Knoxville is used to designate an area in the northeastern corner of Napa County. It was a mining area - there were mercury mines there. Maybe there was a post office in some one's mining office? The Napa County Historical Society may have more information. I'm not aware that there are any remaining residences in the area since the mines closed; much of the land is now owned by BLM, etc.