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Delay tolerant networks (DTNs) characterise a class of emerging networks that suffer from frequent and long-duration partitions. As the storage-carry-forward paradigm is adopted to transfer messages in DTNs, buffer management schemes greatly influence the performance of routing protocols when nodes have limited buffer space. From a network-wide viewpoint, the excessive increase of a single message's copies will exhaust nodes' buffer space, thus reduces the probability of other messages to be buffered and forwarded and leads substantial decrease in their delivery ratio. In this paper, inspired by the law of diminishing marginal utility in economics, we propose a buffer management scheme based on estimated status of messages, e.g., the total number of copies in the network and the dissemination speed of a message. When performing buffer replacement and scheduling, nodes use encounter histories to estimate status of messages and act accordingly: when buffer overflow occurs, messages that have larger estimated number of copies and faster dissemination speed are replaced prior to and forwarded posterior to other messages. Simulation results show that our buffer management scheme can improve delivery ratio and has relative lower overhead ratio compared with other buffer management schemes.