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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 Communication and Message Transport Methods, Client/Server Roles and Terminology 1 2 3 4 SMTP Mail Transaction Process

Connection Establishment and Greeting Exchange

Let’s take a look at these processes in more detail, starting of course with SMTP session establishment. The SMTP sender begins by initiating a TCP connection to the SMTP receiver. The sending SMTP server uses an ephemeral port number, since it is playing the role of the client in the transaction. Assuming that the server is willing to accept a connection, it will indicate that it is ready to receive instructions from the client by sending reply code 220. This is called the “greeting” or “service ready” response. It commonly includes the full domain name of the server machine, the version of the SMTP server software it is running, and possibly other information.

Now, it would be rude for the server acting as a client to just start sending commands to the responding server without saying hello first, wouldn't it? So that's exactly what comes next: the client says “hello”. In the original SMTP protocol, this is done by issuing a HELO command, which includes the domain name of the sending (client) SMTP server as a courtesy. The receiving device then responds back with a return “hello” message using an SMTP reply code 250.

For example, if the SMTP server “smtp.sendersite.org” was making a connection to the SMTP server “mail.receiversplace.com”, it would say:

HELO smtp.sendersite.org

After receiving this, “mail.receiversplace.com” would respond back with a “hello” message of its own, something like this:

250 mail.receiversplace.com Hello smtp.sendersite.org, nice to meet you.

The “chatty” text is of course purely optional; most of the time SMTP communication is between software programs, so all the pleasantries are mostly just programmers having a sense of humor. Still, isn't such politeness a pleasant thing to see in this sometimes difficult world of ours? J

SMTP Communication and Message Transport Methods, Client/Server Roles and Terminology 1 2 3 4 SMTP Mail Transaction Process