Simple Mail Transfer Protocol (SMTP) is an application layer protocol for e-mail communication. It has been adopted as a standard by Internet Engineering Task Force (IETF). SMTP has set conversational and grammatical rules for exchanging messages between connected computers. It has evolved through several revisions and extensions since its formation by Jon Postel in 1981. In SMTP, the sender establishes a full-duplex transmission channel with a receiver. The receiver may be either the ultimate destination or an intermediate forwarding agent. SMTP commands are issued by the sender and are sent to the receiver, which responds to these commands through codes. Each SMTP session between the sender and the receiver consists of three phases namely: connection establishment, mail transactions and connection termination. This paper describes and illustrates the process of e-mail communication through SMTP by issuing the individual SMTP commands directly to transmit e-mail messages. It also describes individual SMTP commands and extensions with practical implementation using a Telnet client.
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