SMTP Commands

Early TCP/IP email 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; see Chapter 87). In fact, you can check the function of an SMTP ...

Get TCP/IP Guide now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.