SMTP Communication and Message Transport Methods, Client/Server Roles and Terminology
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The TCP/IP electronic mail communication model describes the way e-mail messages are conveyed from the sender to the recipient. In most cases, this involves the sender's client machine sending the e-mail to its local SMTP server, which in turn sends it to the recipient's local SMTP server, and finally to the recipient's local host. All of these steps except for the last one are performed by SMTP. In fact, the overall e-mail communication model is largely described by the RFC 821 and 2821 SMTP standards.
The initial communication takes place between the sender's client machine and a local SMTP server that the sender is allowed to access. After submission of the e-mail message, that SMTP server becomes responsible for delivering the message to the SMTP server responsible for the recipient's mailbox. There are two different ways that this can happen.
In the first years of electronic mail, when RFC 821 and its predecessors were initially defined, the Internet was very different than it is today. There was no Domain Name System, and this made electronic mail delivery complex, because there was no way to map a mailbox address to the IP address of the SMTP server that managed that mailbox. Also, there were many proprietary networks connected to the Internet, which meant that it was no always possible for any given system to communicate with any other.
Given this, how could e-mail be delivered? The most common way in the early days of SMTP was through a process called relaying. SMTP routing information was included along with the e-mail address, to specify a sequence of SMTP servers that the mail should be relayed through to get to its destination. For example, if a sender using SMTP server A wanted to send e-mail to someone whose mailbox was on SMTP server Z, they might have needed to specify that the mail be sent through intermediate SMTP servers at sites D, P and U to get there. An SMTP connection would be established from A to D to send the message on one leg of its journey; then it would go from D to P, P to U and then U to Z. The process is analogous to how IP routing works, but at the application layer (actually using IP routing at a lower level, of course.)
You can probably see the problems with this quite easily: it's cumbersome, requires many devices to handle the mail, results in delays in communication, and also requires the communication of source routes between SMTP servers. It was certainly functional, but far from ideal.
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