This paper focuses on the features of two KQML performatives, namely tell and untell, in the context of nonprioritized belief change. Tell allows agents to send beliefs while untell allows agents to send explicit disbeliefs. In a multi agent system, agents have to change their belief when they receive new information from other agents. They may revise or contract their belief state accordingly. The revision action consists of inserting a new belief in a beliefs set while the contraction action consists of managing a set of disbeliefs. Whenever incoming information entails inconsistencies in an agent?s belief state, the agent must either drop some beliefs or refuse the incoming statement. For this, agents consider a preference relation over other agents embedded in the multi agent system and may reject new information based on their belief state and their preference relation. In this article, we survey a logic-based framework for handling messages and (dis)beliefs change. In this context, we formally describe the consequences of tell and untell performatives.