link to X.400


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.

link to X.25 Protocol

X.25 Protocol

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).