In sparse mobile social networks, the moving activity of nodes always happen in a specific area, which corresponds to some specific community. How to guarantee the higher packet delivery ratio while reducing forwarding delay in such networks, is a challenging issue and has not been widely investigated yet. Recently, additional super-node was introduced to ferry packets between the isolated areas. However, most existing solutions assume the super-node is always moving according to the fixed trajectory. In this paper, we put some special mobile nodes in the networks, they are called postmen, and their responsibility is to carry packets for normal nodes which belong to specific communities. Our work focus on the optimization of the moving trajectory by considering the minimum transmission delay. We formulate the optimal issue into semi-Markov Decision Process model. The decision process includes two parts: Packets-choosing strategy and trajectory of packet-ferrying-determination strategy. By maximizing the individual reward of a postman, its optimal trajectory will be found. Furthermore, the proposed solution guarantees the packet delivery ratio and delay for isolated communities. Simulation results show that the proposed packet-ferrying solution outperforms two existing ferrying solutions in terms of packet delivery ratio.