Anonymous Letter is a written note found in Dishonored 2.
I don't give your building three days before it falls to the ground. By that time, you could be sailing to Driscol, on the ship I told you about. The Captain will grant you passage for free if I give him the word. They're leaving this afternoon. All you have to do is pass by that little courtyard behind the Dunwall Courier, and do as I told you. Nobody will know.
Go back to your family, leave Dunwall and forget about the Boyle Company. This is your last chance.