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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 Security Issues 1 2 3 SMTP Replies and Reply Codes

I mentioned in the overview and history of SMTP that early TCP/IP e-mail mechanisms were developed by borrowing techniques and elements from existing application protocols, especially Telnet and FTP. SMTP is an independent protocol, but its heritage can still be seen clearly in a few areas. One of the more obvious of these is in the method by which commands are issued by an SMTP sender and replies returned by an SMTP receiver.

Like FTP, all SMTP commands are sent as plain ASCII text over the TCP connection established between the client and server in an SMTP connection. These commands must end with the two-character “CRLF” sequence that normally terminates ASCII text as required for the Telnet Network Virtual Terminal (NVT). In fact, you can check the function of an SMTP server and even issue commands to it yourself, simply by using Telnet to connect to it on port 25.

All SMTP commands are specified using a four-letter command code. Some commands also either allow or require parameters to be specified. The basic syntax of a command is:

<command-code> <parameters>

When parameters are used, they follow the command code and are separated from it by one or more space characters. For example, the HELO and EHLO commands are specified with the command code, a space character, and then the domain name of the SMTP sender, as we saw in our look at SMTP connection establishment.

SMTP Security Issues 1 2 3 SMTP Replies and Reply Codes