Page 195 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes 12.2.2 Mail Exchange

The SMTP design is based on the model of communication illustrated Figure 12.2. After the
client machine mail request, the sender-SMTP sets a two-way connection with a receiver-SMTP.
The receiver-SMTP may be the destination machine or an intermediate machine (mail gateway).
The sender-SMTP will initiate commands which are replied to by the receiver-SMTP.

Figure 12.2: SMTP Communication

SMTP

User Commands/Mail User
Senders
SMTP
Receiver

File SMTP File
System Replies System

1. The client machine SMTP sets a TCP connection with the destination machine SMTP and
then waits for the server to send a service ready message or a service not available message.
2. HELO (HELO is an abbreviation for hello) is sent and the receiver machine will identify
itself by sending back its domain name. The client machine SMTP uses this to verify if it
reached the right destination SMTP. If the client machine SMTP supports SMTP Service
Extensions, it substitutes an EHLO command in place of the HELO command. A destination
machine SMTP which does not support service extensions responds with a 500 Syntax
error, command unrecognized message. The client machine SMTP then retries with HELO,
or if it cannot transmit the message without one or more service extensions, it should send
a QUIT message. If a receiver-SMTP supports service extensions, it responds with a multi-
line 250 OK message which includes a list of service extensions which it supports.

3. The client machine now initiates the start of a mail transaction by sending a MAIL command
to the destination machine. This command has the reverse-path that is used to report
errors. It should be noted that a path is more than just the user mailbox@host domain
name pair. Besides, it has a list of routing hosts.
4. The next step of the actual mail exchange provides the server SMTP with the destinations
for the message; the message may go to more than one recipient. This is accomplished by
sending one or more RCPT TO:<forward-path> commands. Each of them will receive a
reply 250 OK when the destination is known to the server or a 550. No such user here when
it is not known to the server.

5. When all RCPT commands are sent, the sender forwards a DATA command to notify the
destination machine that the message contents are following. The server replies with 354
Start mail input, end with <CRLF>.<CRLF>. It should be noted that the ending sequence
that the client machine uses to terminate the message data.
6. The client machine now sends the data line by line ending with the 5-character sequence
<CRLF>.<CRLF> line upon which the destination machine acknowledges with a 250 OK
or an appropriate error message when anything went wrong.

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