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TCP/IP Guide by Charles M. Kozierok

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SMTP Connection and Session Establishment and Termination

The delivery of email using SMTP involves the regular exchange of email messages among SMTP servers. SMTP servers are responsible for sending email that users of the server submit for delivery. They also receive email intended for local recipients, or for forwarding or relaying to other servers.

Overview of Connection Establishment and Termination

All SMTP communication is done using TCP. This allows SMTP servers to make use of TCP's many features that ensure efficient and reliable communication. SMTP servers generally must be kept running and connected to the Internet 24 hours a day, seven days a week, to ensure that mail can be delivered at any time. (This is a big reason why most end users ...

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