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 Connection and Session Establishment and Termination 1 2 3 SMTP Special Features, Capabilities and Extensions

The delivery of e-mail message begins with the establishment of an SMTP session between the devices sending and receiving the message. The SMTP sender initiates a TCP connection to the SMTP receiver, and then sends a HELO or EHLO command, to which the receiver responds. Assuming there are no problems, the session is then established and ready for actual e-mail message transactions.

The SMTP mail transaction process itself consists of three steps:

1. Transaction Initiation and Sender Identification: The SMTP sender tells the SMTP receiver that it wants to start sending a message, and gives the receiver the e-mail address of the message's originator.
   
   
2. Recipient Identification: The sender tells the receiver the e-mail address(es) of the intended recipients of the message.
   
   
3. Mail Transfer: The sender transfers the e-mail message to the receiver. This is a complete e-mail message meeting the RFC 822 specification (which may be in MIME format as well).

That's it! So you can see that the word “Simple” in “Simple Mail Transfer Protocol” definitely has at least some merit. Especially when compared with other protocols that claim to be simple, such as SNMP. J

SMTP Connection and Session Establishment and Termination 1 2 3 SMTP Special Features, Capabilities and Extensions