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Author and Publisher, The TCP/IP Guide
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.
Note: The topic on FTP reply codes contains a thorough explanation of the benefits of using these structure numeric reply codes. Since the SMTP system is so similar, I won't duplicate that discussion in this topic. I will, however, fully describe the syntax and semantics of SMTP reply codes here, in order both to keep this section on SMTP self-contained, and to indicate where the differences are between FTP and SMTP reply codes.