(SPOILERS) A letter was sent from the island SPECIFICALLY to Sgt Howie
I wonder how many of Lord Summerisle's people were on the mainland, watching and observing him, before they decided he would be ideal for their sacrifice, as a complete virgin and totally against their pagan religion?
I would imagine the harbour scene, where Howie arrives on the plane, was just the start. They initially refused to send the dinghy, but their suspicions were confirmed when he persisted that they had the right man. From the moment Howie set foot onto the island, he was doomed. Then, they just had to make sure he couldn't escape, had fun with him, mucking him about with Rowan Morrison is alive/is dead/is not on the island over and over again until the day of the sacrifice came.