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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)
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SMTP Connection and Session Establishment and Termination
(Page 4 of 4)
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.
 Key Concept: An SMTP session consists of three basic phases. The session is first established through the creation of a TCP connection and the exchange of identity information between the SMTP sender and receiver using the HELO command. Once established, mail transactions can be performed. When the SMTP sender is done with the session, it terminates it using the QUIT command. If SMTP extensions are supported, the SMTP sender uses the EHLO (extended hello) command instead of HELO, and the SMTP receiver replies with a list of extensions it will allow the SMTP sender to use.
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Version 3.0 - Version Date: September 20, 2005
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