X.400 Connector is a type of connector that is available as an optional component of Microsoft Exchange Server.
X.121 Address also called an international data number (IDN), is an address of an end node (computer or terminal) that is connected to an X.25 public data network.
X.400 is a set of standards defined in 1984 and 1988 by the International Telecommunication Union (ITU) for computer-based handling of e-mail.
Xerox Network Systems is a suite of networking protocols developed by Xerox Corporation’s Palo Alto Research Center (PARC) in the early 1980s.
X.25 is a packet-switching protocol for wide area network (WAN) connectivity that uses a public data network (PDN) that parallels the voice network of the Public Switched Telephone Network (PSTN).