Protocol

Protocol is a common function or a means for unrelated objects to communicate with each other in object-oriented programming. A communications protocol, like those used in email messaging, is a system of digital formats and the rules for exchanging those messages between servers or computing systems. These formats and rules may encompass signaling, authentication, and error detection and correction.

While protocols may vary, a protocol must describe: the messages that are understood by each object; the arguments with which these messages may be supplied; the results possible to each message; the invariants that cannot be altered no matter the state of an object; and the exceptional situations that may be required to be handled by clients to the object.

Mail servers have used the communication protocol: SMTP (simple mail transfer protocol), as well as higher-level IMAP (internet message access protocol) and POP (post office protocol). Each of these protocols provide the scripting details for a broad range of programs on how to handle sending, receiving and storing email messages.