In the data transmission process of delay-tolerant mobile sensor networks, data is easily lost, and the network lifetime decreases due to energy depletion by the nodes. We propose a reliable energy-aware routing protocol, called RER. To ensure the reliability of message transmission, a hop-by-hop retransmission acknowledgement mechanism is introduced in the RER. Second, we design a metric called Reliable Energy Cost Based on Distance (RECBD) to aid RER, which is determined by analysing the distance between the current node and the relay node, the distance between the relay node and the sink node, the current residual energy of the current node, and the link quality. Finally, the message is routed based on the RECBD to improve reliability and reduce energy consumption. The simulation results show that the routing protocol can improve the energy utilization of the sensor nodes and prolong the network lifetime while guaranteeing the delivery ratio and reliability.