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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)

SMTP Commands 1 2 3 4 TCP/IP Electronic Mail Access and Retrieval Protocols and Methods

SMTP communication begins with the establishment of an SMTP session between an SMTP sender (the device acting as a client) and an SMTP receiver (the device acting as a server). All protocol operations then consist of plain ASCII text SMTP commands issued by the sender to the receiver. The receiver analyzes each command, carries out the instruction requested by the sender if possible, and then responds back with a reply to the sender. The reply serves several functions: confirming command receipt; indicating if the command was accepted or not; 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 not just of a string of reply text, but a combination of reply text and a numerical reply code. And as with FTP, these reply codes use three digits to encode various information about the reply, with each digit having a particular significance. The reply code is really the key part of the reply, with the reply text being merely descriptive.

SMTP Commands 1 2 3 4 TCP/IP Electronic Mail Access and Retrieval Protocols and Methods