Congestion control on the Internet has been a challenge issue so far. Since there is no one-size-fits-all scheme for TCP congestion control, many various schemes have been developed to suit different kinds of communication environments. Among them, ECN (Explicit Congestion Notification) is the one which is able to feed the congestion state in the routers along a connection path back to the TCP transmitter. To promote the deployment of ECN and provide a concrete reference parameter to the Internet industry, in this paper, we actually measure the readiness of ECN in mail servers and analyze its deployment rate on the Internet. To clarify the characteristics of ECN in the mail servers, deployment rate of ECN is compared with that of ECN in major web servers. Based on the analysis results, we show some unique features of ECN in mail servers which are quite different from that of web servers.