The history of email entails an evolving set of technologies and standards that culminated in the email systems in use today. Computer-based messaging between users of the same system became possible following the advent of time-sharing in the early 1960s, with a notable implementation by MIT's CTSS project in 1965. Informal methods of using shared files to pass messages were soon expanded into the first mail systems. Most developers of early mainframes and minicomputers developed similar, but generally incompatible, mail applications. Over time, a complex web of gateways and routing systems linked many of them. Some systems also supported a form of instant messaging, where sender and receiver needed to be online simultaneously. In 1971 the first ARPANET network mail was sent, introducing the now-familiar address syntax with the '@' symbol designating the user's system address. Over a series of RFCs, conventions were refined for sending mail messages over the File Transfer Protocol. Several other email networks developed in the 1970s and expanded subsequently. Proprietary electronic mail systems began to emerge in the 1970s and early 1980s. IBM developed...