Message Transfer

The message transfer system is concerned with relaying messages from the originator to the recipient. The simplest way to do this is to establish a transport connection from the source machine to the destination machine and then just transfer the message.

SMTP—The Simple Mail Transfer Protocol

Within the Internet, e-mail is delivered by having the source machine establish a TCP connection to port 25 of the destination machine. Listening to this port is an e-mail daemon that speaks SMTP (Simple Mail Transfer Protocol).

This daemon accepts incoming connections and copies messages from them into the appropriate mailboxes. If a message cannot be delivered, an error report containing the first part of the undeliverable message is returned to the sender.

SMTP is a simple ASCII protocol. After establishing the TCP connection to port 25, the sending machine, operating as the client, waits for the receiving machine, operating as the server, to talk first.

The server starts by sending a line of text giving its identity and telling whether it is prepared to receive mail. If it is not, the client releases the connection and tries again later. If the server is willing to accept e-mail, the client announces whom the e-mail is coming from and whom it is going to.

If such a recipient exists at the destination, the server gives the client the go-ahead to send the message. Then the client sends the message and the server acknowledges it. No checksums are needed because TCP provides a reliable byte stream. If there is more e-mail, that is now sent.

When all the e-mail has been exchanged in both directions, the connection is released. A sample dialog for sending the message of Fig. as shown MIME, including the numerical codes used by SMTP, is shown in Fig.1. The lines sent by the client are marked C:. Those sent by the server are marked S:.

              Figure1. Transferring a message from elinor@abcd.com to carolyn@xyz.com. 

                                                           
A few comments about Fig.1 may be helpful. The first command from the client is indeed HELO. Of the various four-character abbreviations for HELLO, this one has numerous advantages over its biggest competitor.

In Fig1, the message is sent to only one recipient, so only one RCPT command is used. Such commands are allowed to send a single message to multiple receivers. Each one is individually acknowledged or rejected.

To get a better feel for how SMTP and some of the other protocols described in this chapter work, try them out. In all cases, first go to a machine connected to the Internet. On a UNIX system, in a shell, type

telnet mail.isp.com 25
substituting the DNS name of your ISP's mail server for mail.isp.com.

Using ASCII text makes the protocols easy to test and debug. They can be tested by sending commands manually, as we saw above, and dumps of the messages are easy to read. To get around some of these problems, extended SMTP (ESMTP) has been defined in RFC 2821.

Clients wanting to use it should send an EHLO message instead of HELO initially. If this is rejected, then the server is a regular SMTP server, and the client should proceed in the usual way. If the EHLO is accepted, then new commands and parameters are allowed.