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TCP/IP Guide by Charles M. Kozierok

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SMTP Replies and Reply Codes

All SMTP protocol operations consist of the plain ASCII text SMTP commands you saw in Table 77-2, issued by the sender to the receiver. The receiver analyzes each command, carries out the instruction requested by the sender if possible, and then sends a reply to the sender. The reply serves several functions: confirming command receipt, indicating whether or not the command was accepted, and communicating the result of processing the command.

Just as SMTP commands are sent in a manner reminiscent of how FTP internal commands work, SMTP replies are formatted and interpreted in a way almost identical to that of FTP replies. As with FTP, the reply consists of not just a string of reply text, but a combination of reply ...

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