Simple Mail Transfer Protocol

The Simple Mail Transfer Protocol (SMTP) mechanism is used for providing e-mail services to IP devices over the Internet. SMTP is defined in RFC 821. Typically, two mail servers will talk SMTP to exchange mails. After the mails are exchanged, the users can read/retrieve their mail from the mail server. This can be done using any mail client, such as Pine, Eudore, Outlook, and so on, which use different protocols, such as Post office protocol or POP3, to connect to the server. SMTP uses well-known ports TCP port 25 and UDP port 25.

A process or daemon running on a server will use SMTP to send mail to clients. A program called Sendmail is a common tool used for SMTP mail transfer. Recently, a new release of SMTP, called Enhanced SMTP (ESMTP), was developed. You are not required to know this protocol for the written exam.

NOTE The client and SMTP server send various commands when communicating. The most common command is HELO check.

The HELO Check command introduces the calling machine to the receive machine; the client will advertise the mail server its host name. There are numerous other commands. A great resource if you are interested in further details on the Sendmail application is the book "Sendmail," by Bryan Costales and Eric Allman (O'Reilly and Associates, ISBN 1-56592-839-3).

To test if a remote host's SNMP mail is operational and active, you can use Telnet with the defined HELO command.

A summary of other useful SMTP commands is presented for your reference in case you are questioned on these commands during the exam: HELLO (HELO)—Identifies the sender.

MAIL (MAIL)—Initiates a mail transaction in which the mail data is delivered to mailboxes. RECIPIENT (RCPT)—Identifies an individual recipient of the mail data; multiple use of the command is needed for multiple users.

DATA (DATA)—The lines following the command are the mail data in ASCII character codes. SEND (SEND)—Initiates a mail transaction in which the mail data is delivered to one or more terminals.

SEND OR MAIL (SOML)—Initiates a mail transaction in which the mail data is delivered to one or more terminals or mailboxes.

SEND AND MAIL (SAML)—Initiates a mail transaction in which the mail data is delivered to one or more terminals and mailboxes.

RESET (RSET)—The current mail transaction is to be aborted. Any stored sender, recipients, and mail data must be discarded, and all buffers and state tables cleared. The receiver must send an OK reply.

VERIFY (VRFY)—This is to verify if a user exists; a fully specified mailbox and name are returned.

NOOP (NOOP)—Specifies no action other than that the receiver sent an OK reply.

QUIT (QUIT)—The receiver must send an OK reply and then close the transmission channel.

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