This work presents a comprehensive performance study on Internet Service Provider (ISP) mail server, which plays an important role in Internet-based distributed computing. By feeding the SPECmail2001 benchmark into an ISP mail server testbed set up by a commercial mail server software system - MDaemon 5.0.1, we study both networking and I/O performance by varying its user population from 200 to 10,000. The benchmark utilities are adopted for networking analysis while file system traces are collected for I/O measurement. Based on the benchmark study and offline trace analysis, we arrive at several important conclusions for ISP mail servers and give corresponding technical suggestions to improve the performance. First we observed that, in SMTP and POP sessions, the initial network connection setup step usually takes a very long time. Second, I/O latencies typically contribute to 40-55% of the total data transfer time in e-mail requests, especially in a server with large user population support. Third, A group of e-mail messages will easily make a remote recipient server become overloaded.