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TCP/IP Guide by Charles M. Kozierok

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TCP/IP Email Message Communication Model

The purpose of the email system as a whole is to accomplish the transmission of messages from a user of a TCP/IP internetwork to one or more recipients. To accomplish this, a special method of communication is required that makes the email system quite different from that used by most other protocols. To understand what I mean by this, just consider the difference in communication between sending a letter and making a phone call.

Most TCP/IP protocols are analogous to making a phone call in this respect: The sender and the receiver must both be on the network at the same time. You can't call someone and talk to him if he isn't around to answer the phone. (I'm ignoring answering machines and voice mail of ...

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