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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 Termination

When the sending device is finished sending all the e-mail it has to transfer to the receiving device, and done with all other activities, it terminates the session by issuing the QUIT command. This normally results in a 221 “goodbye” message from the SMTP receiver, saying something like “closing transmission channel”. The TCP connection is then terminated.

A server may also terminate prematurely in special cases. If it is given a local command to shut down (for example, due to imminent rebooting of the hardware server upon which it is running), it may respond to any routine command with a 421 response (“Service not available, closing transmission channel”). A server is not supposed to terminate a session simply due to receipt of an invalid command, however, only in special cases where session termination cannot be avoided.

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