SMTP is the standard protocol for mail transfer between servers on the Internet. However, a few other protocols are used for server-server transfer within other mail systems. For instance, both Lotus Notes and Microsoft Exchange, discussed in the following sections, provide email service, among other services. Each of these systems has one proprietary protocol that is used for server-server and server-client mail transfer, as well as other functions. As discussed earlier, both of them are also capable of using SMTP to speak to the Internet.
In addition, the X.400 mail protocol is part of the Open Systems Interconnect (OSI) protocol set. Some mail systems are still running it, and both Lotus Notes and Microsoft Exchange are also capable of using X.400 to speak to other mail systems. On TCP/IP networks, X.400 is run over TCP port 102. This is actually a general-purpose port that is used to provide OSI's Transport Service Access Protocol (TSAP) and that can carry any higher-level OSI protocol. X.400 and X.500 (directory services) happen to be the only higher-level OSI protocols that are still used to any noticeable extent, but opening up port 102 exposes you to all OSI protocols, not just to these.