SMTP servers are able to directly communicate among themselves without involving any other central system in order to help with the message clearing. The messages can travel through several different SMTP servers before they will reach their destination, which is the mailbox of the recipient. When a client starts session with a SMTP server, it provides it with a source route that contains a list of hosts that the mail has to pass through. This route is called a forward path.
In addition, there is also a second route known as traverse path which is designed to return error messages in case of failure in the mail delivery. The best thing about it is that the decentralized system of the mail delivery process is very straightforward and simple. Also, if the SMTP server fails it will not affect the entire system.
Most SMTP servers are negotiating with each other in order to find out which extension is compatible with each. The service extension is divided into four categories such as: authentication and security, delivery, command pipelining and enhanced code. Also in the original RFC, each process has a name according to its service. For example, sender SMTP is known as a client that builds a two-way session with its local server.
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