Message Transfer and SMTP
Message Transfer and SMTP – The message transfer system is connected 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(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 email daemon that speaks SMTP (simple mail transfer protocol). This daemon accept incoming connections and copies message from them into the appropriate mailboxes. If a message cannot be delivered, an error report containing the first part of the establishing the TCP connection to port 25, the sending machine, operating as the client, wits 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 a mail. If it is not, the client releases the connection and tries again later.
I the server is willing to accept email, the client announces whom the email 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 checksum are needed because TCP provide a reliable byte stream. If there is more email, that is now sent. When all the email has exchanged in both directions, the connection is released. Finally, although the syntax of the four character commands from the client is rigidly specified, the syntax of the replies is less rigid. Only the numerical code really counts. Each implementation can put whatever string it wants after the code.
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. On a windows system, click on start then run, and type the command in the dialog box. This command will establish a telnet (i.e, TCP) connection to port 25 on that machine. Port 25 is the SMTP port. You will probably get a response something like this:
Connected to mail.isp.com
Escape character is ‘^]’.
220 mail.isp.com.com Smail#74 ready at thu, 25 sept 2002 13:26 +0200
The first three lines are from telnet telling you what it is doing. The last line is from the SMTP server on the remote machine announcing its willingness to talk to you and accept e-mail.