Electronic Mail
Contents
Email
Email Scenario
SMTP
POP3
IMAP
Electronic Mail (EMAIL)
One of the most popular Internet services is electronic mail (e-mail).
The general architecture of an e-mail system including the three main
components:
User agent (UA)
Message transfer agent (MTA)
Message access agent (MAA)
To explain the architecture of e-mail, four scenarios will be discussed from simple
to complex level.
Electronic Mail (EMAIL) Architecture: First Scenario
UA: User Agent UA: User Agent
Sender Receiver
System
Two user agents are used while the sender and the receiver
of an e-mail are on the same system.
Electronic Mail (EMAIL) Architecture Second Scenario
UA: User Agent UA: User Agent
Sender Receiver
MTA Internet
MTA
Client Server
System’ System’
(mail Server) (mail Server)
When the sender and the receiver of an e-mail are on
different systems, we need two UAs and a pair of
(Message transfer agent )MTAs (client and server).
Electronic Mail (EMAIL) Architecture Third Scenario
MTA
UA Sender Client
MTA
Server UA: User Agent
Receiver
LAN
or
WAN
MTA Internet
MTA
Client Server
The sender is
connected to the System
mail server via a
System
LAN or a WAN. (mail Server) (mail Server)
Electronic Mail (EMAIL) Architecture Fourth Scenario
Receiver
MAA UA
UA: User Agent MTA Client
Client
Sender MAA
MTA
Server Server
LAN
or LAN
WAN or
WAN
MTA Internet MTA
The sender and receiver Client Server
both are connected to the
mail server via a LAN or a
System’ System’
WAN.
(mail Server) (mail Server)
Push versus pull in electronic email
Services of user agent
It provides service to the user to make the process of sending
and receiving a message easier.
Electronic Mail (EMAIL) Addresses
To deliver mail, a mail handling system must use an addressing
system with unique addresses.
In the Internet, the address consists of two parts: a local part and a
domain name, separated by an @ sign
Electronic Mail (EMAIL) Addresses
Local Part: Defines the name of a special file, called the user mailbox, where all the mail
received for a user is stored for retrieval by the message access agent.
Domain Name: The second part of the address is the domain name. An organization usually
selects one or more hosts to receive and send e-mail; the hosts are sometimes called mail
servers or exchangers. The domain name assigned to each mail exchanger either comes from the
DNS database or is a logical name (for example, the name of the organization).
EMAIL Message Transfer Agent: SMTP
The actual mail transfer is done through message transfer agents.
To send mail, a system must have the client MTA, and to receive mail, a
system must have a server MTA.
The formal protocol that defines the MTA client and server in the Internet
is called the Simple Mail Transfer Protocol (SMTP).
SMTP is used two times, between the sender and the sender’s mail
server and between the two mail servers.
Mechanism of mail transfer by SMTP
Commands and Responses
SMTP uses commands and responses to transfer messages
between an MTA client and an MTA server
Commands- Commands are sent from the client to the server.
The format of a command is shown in Figure
SMTP defines 14 commands.
The first five are mandatory; every
implementation must support these five
commands. The next three are often
used and highly recommended. The last
six are seldom used.
Mechanism of mail transfer by SMTP
Responses
Responses are sent from the server to the client. A response is a three
digit code that may be followed by additional textual information
Mail Transfer Phases
The process of transferring a mail message occurs in three phases:
connection establishment, mail transfer, and connection termination.
Message Access Agent: POP and IMAP
The first and the second stages of mail delivery use SMTP.
However, SMTP is not involved in the third stage because SMTP
is a push protocol; it pushes the message from the client to the
server. In other words, the direction of the bulk data (messages) is
from the client to the server.
On the other hand, the third stage needs a pull protocol; the
client must pull messages from the server. The direction of the
bulk data is from the server to the client.
The third stage uses a message access agent.
Currently two message access protocols are available: Post
Office Protocol, version 3 (POP3) and Internet Mail Access
Protocol, version 4 (IMAP4).
POP3
Post Office Protocol, version 3 (POP3) is simple and limited in
functionality.
The client POP3 software is installed on the recipient computer;
the server POP3 software is installed on the mail server.
Mail access starts with the client when the user needs to download
e-mail from the mailbox on the mail server.
The client opens a connection to the server on TCP port 110.
It then sends its user name and password to access the mailbox.
The user can then list and retrieve the mail messages, one by one.
POP3 has two modes: the delete mode and the keep mode
POP3
In the delete mode, the mail is deleted from the mailbox after each
retrieval.
In the keep mode, the mail remains in the mailbox after retrieval.
The delete mode is normally used when the user is working at her
permanent computer and can save and organize the received
mail after reading or replying.
The keep mode is normally used when the user accesses her mail
away from her primary computer (e.g., a laptop).
The mail is read but kept in the system for later retrieval and
organizing.
IMAP4
Another mail access protocol is Internet Mail Access Protocol,
version 4 (IMAP4).
IMAP4 is similar to POP3, but it has more features; IMAP4 is
more powerful and more complex.
POP3 is deficient in several ways. It does not allow the user to
organize her mail on the server; the user cannot have different
folders on the server. (Of course, the user can create folders on
her own computer.)
In addition, POP3 does not allow the user to partially check the
contents of the mail before downloading.
IMAP4
IMAP4 provides the following extra functions:
❏ A user can check the e-mail header prior to downloading.
❏ A user can search the contents of the e-mail for a specific string of
characters prior to downloading.
❏ A user can partially download e-mail. This is especially useful if
bandwidth is limited and the e-mail contains multimedia with high
bandwidth requirements.
❏ A user can create, delete, or rename mailboxes on the mail server.
❏ A user can create a hierarchy of mailboxes in a folder for e-mail
storage.
References
Behrouz A Forouzan, “Data Communication and Networking”
