SMTP (Simple Mail Transfer Protocol) is the standard mechanism by which mail is delivered on the Internet and on TCP/IP networks. Most PC-based mailers which use the TCP/IP network protocols use SMTP to send mail, but use another protocol called POP3 to receive mail. SMTP is Internet's standard host-to-host mail transport protocol and traditionally operates over TCP, port 25. In other words, a UNIX user can type telnet hostname 25 and connect with an SMTP server, if one is present. 
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SMTP uses a style of asymmetric request-response protocol popular in the early 1980s, and still seen occasionally, most often in mail protocols. The protocol is designed to be equally useful to either a computer or a human, though not too forgiving of the human. From the server's viewpoint, a clear set of commands is provided and well-documented in the RFC. For the human, all the commands are clearly terminated by newlines and a HELP command lists all of them. From the sender's viewpoint, the command replies always take the form of text lines, each starting with a three-digit code identifying the result of the operation, a continuation character to indicate another lines following, and then arbitrary text information designed to be informative to a human.
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If mail delivery fails, sendmail (the most important SMTP implementation) will queue mail messages and retry delivery later. However, a backoff algorithm is used, and no mechanism exists to poll all Internet hosts for mail, nor does SMTP provide any mailbox facility, or any special features beyond mail transport. For these reasons, SMTP isn't a good choice for hosts situated behind highly unpredictable lines (like modems). A better-connected host can be designated as a DNS mail exchanger, then arrange for a relay scheme. Currently, there two main configurations that can be used. One is to configure POP mailboxes and a POP server on the exchange host, and let all users use POP-enabled mail clients. The other possibility is to arrange for a periodic SMTP mail transfer from the exchange host to another, local SMTP exchange host which has been queuing all the outbound mail. Of course, since this solution does not allow full-time Internet access, it is not too preferred.
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RFC 1869 defined the capability for SMTP service extensions, creating Extended SMTP, or ESMTP. ESMTP is by definition extensible, allowing new service extensions to be defined and registered with IANA. Probably the most important extension currently available is Delivery Status Notification (DSN), defined in RFC 1891.
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