SMTP server

1. At the sender's end, an SMTP server takes the message sent by a user's computer.

2. The SMTP server at the sender's end then transfers the message to the SMTP server of the recipient.

3. The SMTP server at the recipient's end then takes the email message and stores it in the appropriate user's mailbox.

SMTP is actually quite simple. The communication between a client and server using SMTP consists of human understandable ASCII text. We shall first describe the steps and then list the actual interaction steps. Here, client means sender, and server means receipent.

1. The process starts with the client sending a TCP connection request to the server.

2. The server sends back a READY FOR MAIL reply, indicating that it can accept an email message from the client.

3. The client then sends a HELO (abbreviation of HELLO) command to the server, and identifies itself.

4. The server then sends back an acknowledgement in the form of its own DNS name.

5. The client can now send one or more email messages to the server. The email transfer begins with a MAIL command that identifies the sender.

6. The server allocates buffers to store the incoming email message, and sends back an OK response to the client. The server also sends back a return code of 250, which essentially means OK. The reason both OK and a return code of 250 are sent back is to help both humans and application programs understand the server's intentions (humans prefer OK, application programs prefer a return code such as 250).

7. The client now sends the list of the intended recipients of the email message by one or more RCPT commands (one per recipient). The server must send back a 250 OK or 550 No such user here reply back to the client for each recipient.

8. After all RCPT commands, the client sends a DATA command, informing the server that the client is ready to start transmission of the email message.

9. The server responds with a 354 Start mail input message, indicating that it is ready to accept the email message. It also tells the client what identifiers it should send to signify that the message is over.

10. The client sends the email message and when it is over, sends the identifier provided by the server to indicate that its transmission is over.

11. The server sends back a 250 OK response.

12. The client sends a QUIT command to the server.

13. The server sends back a 221 Service closing transmission channel message, indicating that it is also closing its portion of the connection.

The actual interaction using SMTP as just described, is shown in Figure 5.18. Here, a user Atul at host yahoo.com sends an email to users Jyoti and Sarika at host excite.com. The SMTP client software on the host yahoo.com establishes a TCP connection with the SMTP server software on the host excite.com (not shown in the figure). Thereafter, the exchange of messages happens as shown below (S indicates messages sent by the server to the client, and C indicates messages from the client to the server). Also note that the server has informed the client that it would like to see a identifier when the client's transmission is over. Accordingly, the client sends this when its transmission is over.

S: 220 excite.com Simple Mail Transfer Service Ready
C: HELO yahoo.com
S: 250 excite.com
C: MAIL FROM: S: 250 OK C: RCPT TO: <,Jyoti 0 excite.com>
S: 250 OK
C: RCPT TO:
S: 250 OK C: DATA S: 354 Start mail input; end with
C: ... actual contents of the message ...
C: ......
C: ......
C:
S: 250 OK
C: QUIT S: 221 excite.com Service closing transmission channel

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