Sonning river letterbox proves a challenge for posting The appearance of a red letterbox in the middle of a bridge over the River Thames has created quite a mystery. Residents in Sonning-on-Thames in Berkshire started noticing the box in recent weeks, but say it can only be reached by river users. Royal Mail say the letterbox is not operational and have no idea how it got there. BBC South Today's Joe Campbell took to the water in an attempt to make a special delivery.