In this paper we discuss the adaptive User Model component of the AthosMail system, and describe especially the Cooperativity Model which produces recommendations for the appropriate explicitness of the system utterances, depending on the user’s observed competence levels. The Cooperativity Model deals with the system’s dialogue control and explicitness of the given information: these two aspects affect the system’s interaction capabilities and thus naturalness of the dialogue as a whole. The model consists of an offline and an online version, which use somewhat different input parameters, due to their different functionality in the system.