Early interoperability among independent systems included:
ARPANET, a forerunner of the Internet, defined protocols for dissimilar computers to exchange email.
uucp implementations for Unix systems, and later for other operating systems, that only had dial-up communications available.
CSNET, which initially used the UUCP protocols via dial-up to provide networking and mail-relay services for non-ARPANET hosts.
Novell developed the Message Handling System (MHS) protocol[but abandoned it after purchasing the non-MHS WordPerfect Office (renamed Groupwise)
The Coloured Book protocols on UK academic networks until 1992
X.400 in the 1980s and early 1990s was promoted by major vendors, and mandated for government use under GOSIP, but abandoned by all but a few in favor of Internet SMTP by the mid-1990s.