Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 242
SMTP Flow

The SMTP design is based on the following model of communication:

Once you have filled out the header and body section of your mail message, the sender SMTP establishes two-way communication to a receiver SMTP. The receiver SMTP may be either the ultimate destination or a transient stop on the way to the final destination. Commands are sent to the receiver by the sender SMTP and SMTP replies are sent from the receiver SMTP to the sender SMTP in response to each of the commands.

Once two-way communication has been established, a series of commands (of which you can see operate using some mail applications) are issued. The sender SMTP will send a HELLO (HELO) command identifying who it is using its domain name to the receiver. The receiver acknowledges this with a reply using its domain name. Next, the server issues a MAIL command to the receiver. In this will be the identification of the person (place, or thing) sending the mail. The receiver acknowledges this with an OK. The sender SMTP then sends a RCPT command to the receiver, using the intended receiver name as an argument. Each recipient in the list is sent to the receiver one at a time, and each time the receiver acknowledges with an OK for those recipients that it knows about. For those that it does not know about (different domain name), it will send back a different reply that it is forwarding the message on. For any intended recipients received from the SMTP sender for which it has no account, the receiver will reply to the sender that no such user(s) exists. After the intended recipients have been ACK’d or NACK’d, the SMTP sender sends the DATA command and the SMTP receiver will OK this and indicate what the end of message identifier should be. Once this is received (the ending identifier), the SMTP receiver will reply with an OK. Notice that all data is received, the ending identifier is received, and then a reply message is sent by the receiver.

SMTP Flow

If everything went okay, the sender ends the connection with a QUIT command. The SMTP receiver will reply indicating that the communication channel is closed. The minimum commands that a receiver must support are HELO, MAIL, RCPT, DATA, RSET, NOOP, and QUIT.

Depending on the mail program that you use, the transaction between a recipient and sender of mail has been the same since RFC 821 was written. The interface allows you to complete the mail message, filling in the header (addresses and subject) and the body (text) of the letter. When you press the Send button, the following transaction takes place. Some mail programs actually place the mail commands and state numbers on display while the transaction is taking place. It should be noted here that sending mail is immediate. It may get queued for a small length of time on different routers, and transient mail servers, but not for long. This is for the transport of mail. Most electronic mail today is sent via SMTP and will reside on your mail server host until you retrieve it using POP (discussed next). Today, retrieving your mail does not mean that you have to run the SMTP protocol. A server host will accept mail messages directed to you on your behalf. Then you can sign on any time you want and retrieve your mail.