Due to the uncertainty of transmission opportunities between mobile nodes, Delay Tolerant Networks (DTN) routing exploits the opportunistic forwarding mechanism. Optimal policies for opportunistic forwarding are crucial to maximizing the message delivery probability while reducing the forwarding cost. In this paper, we investigate the optimal opportunistic forwarding policies for the energy- constrained DTN by introducing a continuous time Markov framework to model the message dissemination. Based on this framework, we obtain the optimal static and dynamic policies by theoretical analysis and proof. Through extensive numerical results, we conclude that the performance of the optimal dynamic policy is better than that of the static policy. Among the dynamic policies, we find that the negative power function based optimal policy provides the best performance.