Sarah -
I'm not sure what you have heard, but as far as I know the only letters were sent from ports Titanic stopped at in Ireland and France, before it left for the North Atlantic. From out at sea, Titanic could only send wireless messages. Also, I believe the Cunard ocean liner, Mauretania, was the fastest "traveling" ship at that time, and for many years thereafter. Titanic had a post office, in the bottom of the ship. Of course, the mail would have been delivered in New York, and sent out from there.