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Figure 24-2: Alice's mail server transfers Alice's message to Bob's mail server It is important to observe that SMTP does not use intermediate mail servers for sending mail, even when the two mail servers are located at opposite ends of the world If Alice's server is in Hong Kong and Bob's server is in Mobile, Alabama, the TCP "connection" is a direct connection between the Hong Kong and Mobile servers In particular, if Bob's mail server is down, the message remains in Alice's mail server and waits for a new attempt -- the message does not get placed in some intermediate mail server Let's now take a closer look at how SMTP transfers a message from a sending mail server to a receiving mail server We will see that the SMTP protocol has many similarities with protocols that are used for face-to-face human interaction First, the client SMTP (running on the sending mail server host) has TCP establish a connection on port 25 to the server SMTP (running on the receiving mail server host) If the server is down, the client tries again later Once this connection is established, the server and client perform some application-layer handshaking Just as humans often introduce themselves before transferring information from one to another, SMTP clients and servers introduce themselves before transferring information During this SMTP handshaking phase, the SMTP client indicates the email address of the sender (the person who generated the message) and the email address of the recipient Once the SMTP client and server have introduced themselves to each other, the client sends the message SMTP can count on the reliable data transfer service of TCP to get the message to the server without errors The client then repeats this process over the same TCP connection if it has other messages to send to the server; otherwise, it instructs TCP to close the connection Let us take a look at an example transcript between client (C) and server (S) The host name of the client is crepesfr and the host name of the server is hamburgeredu The ASCII text prefaced with C: are exactly the lines the client sends into its TCP socket; and the ASCII text prefaced with S: are exactly the lines the server sends into its TCP socket The following transcript begins as soon as the TCP connection is established: S: C: S: C: S: C: S: C: S: C: C: C: 220 hamburgeredu HELO crepesfr 250 Hello crepesfr, pleased to meet you MAIL FROM: <alice@crepesfr> 250 alice@crepesfr Sender ok RCPT TO: <bob@hamburgeredu> 250 bob@hamburgeredu Recipient ok DATA 354 Enter mail, end with "" on a line by itself Do you like ketchup How about pickles
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file:///D|/Downloads/Livros/computa o/Computer%20Neown%20Approach%20Featuring%20the%20Internet/smtphtm (3 of 14)20/11/2004 15:51:55
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S: 250 Message accepted for delivery C: QUIT S: 221 hamburgeredu closing connection In the above example, the client sends a message ("Do you like ketchup How about pickles ") from mail server crepesfr to mail server hamburgeredu The client issued five commands: HELO (an abbreviation for HELLO), MAIL FROM, RCPT TO, DATA, and QUIT These commands are self explanatory The server issues replies to each command, with each reply having a reply code and some (optional) English-language explanation We mention here that SMTP uses persistent connections: if the sending mail server has several messages to send to the same receiving mail server, it can send all of the messages over the same TCP connection For each message, the client begins the process with a new HELO crepesfr and only issues QUIT after all messages have been sent It is highly recommended that you use Telnet to carry out a direct dialogue with an SMTP server To do this, issue telnet serverName 25 When you do this, you are simply establishing a TCP connection between your local host and the mail server After typing this line, you should immediately receive the 220 reply from the server Then issue the SMTP commands HELO, MAIL FROM, RCPT TO, DATA, and QUIT at the appropriate times If you Telnet into your friend's SMTP server, you should be able to send mail to your friend in this manner (ie, without using your mail user agent)
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