pauloaguia: "But in correspondance chess (and other games) usually the help of books, move databases or even computers is not forbidden. Some of those (if not all) are even good ways to learn how to play the game. And since it can't be controlled anyway..."

This is exactly,what I have said many times before.In addition,I would say this is even the reason, why a correspondence chessplayer plays the game so slowly: He wants study his moves in the books,in the databases and in his engines to decide about one move rightfully. Which other reason should he have to suspend his games over and over?

And if I hear people saying :I want not play against a computer!
Perhaps you have played many times against a computer in the past without knowing it?
But mostly it is a mixture between human and machine.