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SMTP - RFC 821

The SMTP protocol has two different uses: Delivering mail from the user's mail program to his server and delivering mail from this mail server to the server which stores the destination user's account. Both actions, different as they may seem, handle network communication in the exact same way: The client[*] requests a connection to the server[*] using TCP port 25. Once the connection is established, the server expects a sequence of commands (defined by RFC 821[6]), and, if its configuration does not instruct him to do otherwise, accepts and delivers the requested messages.

SMTP is a line-oriented protocol. Requests are sent by the client as a text line (or block of text lines, in the DATA part), and their answers are returned to the client with a status number and, optionally, an explanation. The status number is 200-299 for successful operations, 300-399 to indicate further input is expected, 400-499 to signal failure to execute a request and 500-599 to report an error in the request.



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Gunnar Wolf
2001-03-12