Page 193 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes attempts to send message again later on. If a message event then cannot be delivered in specified
days, the MTA returns an error to the user.
Briefly, when there is an outgoing mail, the SMTP client will connect to the SMTP server and
sends the mail to the remote server. It uses simple and text-based protocol for one or more
destinations of a message. SMTP server also allows telnet service. SMTP can be considered as a
complement of UUCP. Machines connected together could very well transfer e-mails using
UUCP but not the machine connected across a network all the time.

SMTP is also concerned with transferring mail from one MTA to another. The SMTP protocol is
quite simple. It uses the query response model and only a few message types are defined. The
other complex work is handled by TCP. SMTP commands consist of human-readable ASCII
strings. A single TCP connection is used to serially process a set of message exchanges between
a pair of hosts. SMTP never authenticates a sender. Initially, SMTP was implemented using
Sendmail as the mail transfer agents in client server model. Subsequently, standard for binary
file were included in addition to purely ASCII text-based standard. Multipurpose Internet Mail
Extensions (MIME) standard were used to encode binary files for transfer through SMTP, which
has now become a standard with its varied version. SMTP along with Post office Protocol (POP3)
or Internet Message Access Protocol (IMAP) protocols allow retrieving mail from mail server.

Did u know? In other word, SMTP is a push kind of protocol while POP3 and IMAP are pull
protocol.
Internet mail has an important advantage over other mail systems, for example, uucp or bitnet
because Internet mail system provides an end-to-end reliable delivery system. In contrast to
other mail systems, in the Internet mail system, all mail addresses have the same form: local-
part@domain-name.

Self Assessment

Fill in the blanks:

1. SMTP commands consist of human-readable ……………. strings.
2. A mailbox is associated with one login id within a ………………… to store the e-mails of
the user.

3. ………………………… refers to an alias for a set of recipients.
4. DNS server maintains database in both the form that is textual as well as …………………..
notations.

5. The ……………….. can accommodate almost all kinds of organizations by allowing each
group to choose between geographical or organizational naming hierarchies.
6. ………………….. is a push kind of protocol while POP3 and IMAP are pull protocol.

12.2.1 Simple Mail Transfer Protocol (SMTP)

Electronic mail (E-mail) is considered the most widely used TCP/IP application. The Internet
mail protocols enable a client machine to exchange mail and message between TCP/IP hosts.
Three standard protocols are applied to provide such mail application. The SMTP is one of them.
The three standards are given below:
1. SMTP: It is a standard for exchange of mail between two computers (STD 11/RFC 821),
which specifies the protocol used to send mail between TCP/IP hosts.

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