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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

The delivery of electronic mail using the Simple Mail Transfer Protocol (SMTP) involves the regular exchange of e-mail messages between SMTP servers. SMTP servers are responsible for sending e-mail that users of the server submit for delivery. They also receive e-mail either intended for local recipients, or in some cases for forwarding or relaying to other servers.

All SMTP communication is done using the TCP. This allows SMTP servers to make use of TCP's many features that ensure efficient and reliable communication. SMTP servers generally must be kept running and connected to the Internet 24 hours a day, seven days a week, to ensure that mail can be delivered at any time. (This is a big reason why most end-users employ access protocols such as POP3 to access their received e-mail rather than running their own SMTP servers.) The server listens continuously on the SMTP server port, well-known port number 25, for any TCP connection requests from other SMTP servers.

As explained in the previous topic, an SMTP server that wishes to send e-mail normally begins with a DNS lookup of the MX record corresponding to the domain name of the intended recipient's e-mail address, to get the name of the appropriate SMTP server. This name is then resolved to an IP address; for efficiency, this IP address is often included as an Additional record in the response to the MX request, to save the sending server from needing to perform two explicit DNS resolutions.

The SMTP sender then establishes a SMTP session with the SMTP receiver. Once the session is established, mail transactions can be performed, to allow mail to be sent between the devices. When the SMTP sender is done, it terminates the connection. All of these processes involve specific exchanges of commands and replies, which are illustrated in Figure 304.

Figure 304: SMTP Transaction Session Establishment and Termination

An SMTP session begins with the SMTP sender establishing a TCP connection to the SMTP receiver. The receiver sends a ready message; the sender sends a HELO or EHLO command, to which the receiver responds. Assuming no difficulties, the session is established and mail transactions take place. When the sender is done, it sends a QUIT command; the receiver responds with a 221 reply and closes the session.

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

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