Take the boat from the dock near Huntress Kella Nightbow to Auberdine. Once you have arrived in Auberdine, take the Letter of Introduction to Thundris Windweaver.
If you wait out on the dock, you'll see the boat arriving from the port of Auberdine in Darkshore. Once it has offloaded its passengers, you'll be able to board.
After a short journey, the boat will put in at Auberdine and you'll be able to deliver your message.
It's probably best to take this to Thundris Windweaver, an old friend of mine and a well-respected citizen of the town.