A water feed device for feeding water to a moistener for moistening envelope flaps that is incorporated into a mail handling machine, the water feed device comprising a reserve stock of water from which a quantity of water flows to a reservoir of water, said quantity of water being suitable for imbibing a piece of foam dipping in said reservoir of water, said device having at least two distinct water feed circuits for feeding water separately to two separate compartments of the reservoir of water from two isolated compartments of the reserve stock of water in a manner such as to define two distinct levels of water for the two isolated compartments of the reserve of water, and thus to define two different heights of water in the two corresponding separate compartments of the reservoir of water.