From A Wiki of Ice and Fire
The letter was addressed to "Bastard" and sealed with pink sealing wax (which is used by House Bolton), the letters contents were as follows...
|“||Your false king is dead bastard. He and all his host were smashed in seven days of battle. I have his magic sword. Tell his red whore.
Your false king's friends are dead. Their heads upon the walls of Winterfell. Come see them, bastard. Your false king lied, and so did you. You told the world you burned the King-Beyond-the-Wall. Instead you sent him to Winterfell to steal my bride from me.
I will have my bride back. If you want Mance Rayder back, come and get him. I have him in a cage for all the north to see, proof of your lies. The cage is cold, but I have made him a warm cloak from the skins of the six whores who came with him to Winterfell.
I want my bride back. I want the false king's queen. I want his daughter and his red witch. I want this wildling princess. I want his little prince, the wildling babe. And I want my Reek. Send them to me, bastard, and I will not trouble you or your black crows. Keep them from me, and I will cut out your bastard's heart and eat it.