SMTP is the standard protocol for sending emails between servers. Under SMTP, a client SMTP process opens a TCP connection to a remote server SMTP process and sends mail across the connection. The server listens on port 25 for connections. When a connection is made, the two processes execute a simple request-response dialogue defined by SMTP to transmit sender and recipient addresses and the email message itself. Mail is then forwarded to remote servers or delivered locally. POP3 and IMAP allow users to download stored mail from the local server.

1. Simple Mail Transfer Protocol (SMTP)
SMTP is used as the common mechanism for transporting electronic mail among different
hosts within the transmission control protocol/Internet protocol (TCP/IP) suite. It is an
application layer protocol. Under SMTP, a client SMTP process opens a TCP connection to a
server SMTP process on a remote host and attempts to send mail across the connection.
The server SMTP listens for a TCP connection on a specific port (25), and the client SMTP
process initiates a connection on that port (Cisco SMTP, 2005). When the TCP connection is
successful, the two processes execute a simple request–response dialogue, defined by the
SMTP protocol (see RFC 821 for details), in which the client process transmits the mail
addresses of the originator and the recipient(s) for a message. When the server process
accepts these mail addresses, the client process transmits the e-mail instant message. The
message must contain a message header and message text (“body”) formatted in
accordance with RFC 822. Mail that arrives via SMTP is forwarded to a remote server, or it is
delivered to mailboxes on the local server. POP3 or IMAP allow users download mail that is
stored on the local server.
 SMTP is application level protocol.
 SMTP is connection oriented protocol.
 SMTP is text based protocol.
 It handles exchange of messages between e-mail servers over TCP/IP network.
 Apart from transferring e-mail, SMPT also provides notification regarding incoming
mail.
 When you send e-mail, your e-mail client sends it to your e-mail server which further
contacts the recipient mail server using SMTP client.
 These SMTP commands specify the sender’s and receiver’s e-mail address, along
with the message to be send.
 The exchange of commands between servers is carried out without intervention of
any user.
 In case, message cannot be delivered, an error report is sent to the sender which
makes SMTP a reliable protocol.
Client
Sender
TCP connection
(SMTP commands,
replies and mail)
Receiver Server
Figure 1: The basic simple mail transfer protocol (SMTP) model.
User at a
terminal
User at a
terminal
User Agent
User
Agent
Message
Transfer Agent
Message
Transfer Agent
Queue of mail
to be sent
User
Mailboxes

2. Post Office Protocol (POP3)
What is Post Office Protocol?
The Post Office Protocol (POP3) is an Internet standard protocol used by local email software clients
to retrieve emails from a remote mail server over a TCP/IP connection. Since the first version was
created in 1984, the Post Office Protocol (currently at Version 3) has since became one of the most
popular protocols and is used by virtually every email client to date. Its popularity lies in the
protocol’ssimplicitytoconfigure,operate andmaintain.
Email servers hosted by Internet service providers also use POP3 to receive and hold emails intended
for their subscribers. Periodically, these subscribers will use email client software to check their
mailbox onthe remote serveranddownloadanyemailsaddressedtothem.
Once the email client has downloaded the emails, they are usually deleted from the server, although
some email clients allow users to specify that mails be copied or saved on the server for a period of
time.
Email clients generally use the well-known TCP port 110 to connect to a POP3 server. If encrypted
communication is supported on the POP3 server, users can optionally choose to connect either by
using the STLS command after the protocol initiation stage or by using POP3S, which can use the
Transport Layer Security (TLS) or Secure Sockets Layer (SSL) on TCP port 995 to connect to the
server.
How does Post Office Protocol Work?
When a user checks for new email, the client will connect to the POP3 server. The email client then
supplies its username and password to the server for authentication. Once connected, the client
issues a series of text-based commands to retrieve all email messages. It then stores these
downloaded messages on the user’s local system as new emails, deletes the server copies and
disconnectsfromthe server.
By default, the server emails are deleted once they are retrieved. As a result, the emails are tied to
that particular machine and it would not be possible to access the same emails from an email client
on another machine. The user could get around this issue by configuring the email client settings to
leave acopy of the emailsonthe server.
POP frees up mailbox space on the server because emails and attachments are downloaded and
deleted at server-end whenever the email client checks for new mails. Offline email messages stored
in the user’s computer do not have mailbox size limitations except for the hard drive storage
capacity of the PC. One drawback of POP3 mail accounts is that it is difficult for the user to export
mailsif theydecide toswitchemail programsorcomputersystems.
Advantagesof POP3:
- Emailsare downloadedtothe user’scomputer.Messagescanbe readwhenuserisoffline.
- Openingattachmentsisquickandeasyastheyare alreadydownloaded.
- Lessserverstorage space required;all emailsare storedonlocal machine.
- Storage capacityof emailslimitedbythe size of yourharddisk.
- Verypopular,easytoconfigure anduse.
Disadvantagesof POP3:
- Emailscannotbe accessedfromothermachines(unlessconfiguredtodoso).
- Exportingthe local mail foldertoanotheremail clientorphysical machine canbe difficult.
- Email folderscanbecome corrupted,potentiallylosingthe entiremailbox atonce.

3. - Email attachments may contain viruses which may expose the user’s PC to harm if they are opened
locallyandtheirvirusscannerisunable todetectthem.
InternetMessage AccessProtocol (IMAP4)
Introduction
Internet Message Access Protocol (IMAP) and Post Office Protocol (POP3) are protocols used for
email retrieval and they are in-use by almost every modern mail clients and servers. Previously, I
have written an article about Post Office Protocol. This article will focus on what IMAP is,its features
and the difference betweenthese twoprotocols.
What is IMAP (Internet Message Access Protocol)?
IMAP (Internet Message Access Protocol) is a standard protocol used by an email client for accessing
emails that are stored on a remote mail server. Email clients which use IMAP can use either port 143
to establish a non-secured connection, or port 993 if they require to connect securely via IMAP over
SSL (IMAPS). Designed by Mark Crispin in 1986, IMAP has gone through several revisions and its
latestversion –IMAP Version4Revision1(IMAP4rev1) wasdefinedbyRFC3501 in2003.
The difference between IMAP and POP, and other Unique Features of the
IMAP Protocol
IMAP supports both online and offline operation modes. It is an alternative method to POP3 for
receiving email and was designed to address the weak points in POP as well as to provide more
delivery features and email management. While POP3 email clients download, delete and store the
messages offline, IMAP email clients generally store all their messages and folders centrally on the
server and the client manages these remote mail messages by sending a series of commands to the
server.
IMAP mail clients may appear to be managing the messages locally, but they are in fact accessing the
messages online. A user can manage the same mailbox through multiple clients in any configured
computer as long as there is an Internet connection and the mail server is running. By storing the
mail messages online, a user does not have to worry that his email will be lost in the event of a
systemcrashor stolenPCetc.
IMAP email accounts can also be configured to download a cached copy of the message for offline
use. This is different from the POP3 protocol in that IMAP copies the message onto the mail client,
whereas POP3 moves the email to the local client instead. With IMAP, the remote mail server will
retain a master copy of the email and POP3 will delete the server copy of the email after download.
IMAP clientuserscansafelydelete the offline copywithoutaffectingthe mastercopyonthe server.
A feature of the IMAP protocol are “flags” which allows mail clients to keep track of messages that
are unread/read, replied to or deleted. By storing the flags on the remote server, different clients
connected to the same mailbox could detect changes to the message state that were made by other
clients.
Management of multiple mailboxes (also known as folders) on the remote server is supported.
IMAP4 clients use this feature to keep the Inbox organized. Mailboxes can be created, renamed or
deleted. Users can also move messages between mailboxes.A mail client using the IMAP protocol on
one machine could create a mailbox on the remote server, which then becomes visible to all other
mail programs configured with the same IMAP email account. As mailboxes are synced with all mail
clients, moving and organizing messages around will be automatically visible to those clients. IMAP
mailboxes can be subscribed and/or unsubscribed to keep only the important folders for easy
content access which helps to keep the mails and folders organized. Unsubscribed folders are not
deleted from the server but they will not be displayed or included in search from within the mail
client.

4. The IMAP protocol allows clients to fetch and preview only the headers of the message which is
beneficial to slow or limited data connections. Users can also decide to download (or not) large
attachmentsovera slowconnection,allowingthemtosave bandwidthontheirdata plans.
Over time, mail archives will grow in size on the server and IMAP users may need to be mindful of
their email storage space as compared to those POP users. However, this has become less of an
issue as advanced technology and shrinking storage cost result in email service providers like Gmail
and Yahoo who provide ample storage space for their email clients, although increasingly large
quantitiesof email withheavyattachmentscanquicklyfill upthe assignedstorage space.
Advantages of IMAP:
- Email isavailable toanymachine thathas a clientconfiguredwiththe same IMAPaccount
- Email messagesare notlostinthe eventof systemcrashor stolenPCas theyare storedonline
- Abilitytosetmessage statusflags
- Remote mailbox management(list/create/rename/delete)
- Supportforsimultaneousupdateinsharedfoldersoverdifferentclients
- Selectivesubscriptionof relevantmailboxes
- Freedomtopreviewonlypartsof the message withoutdownloadingthe whole email
- Userscan decide whentodownloadattachments
Disadvantages of IMAP:
- New incoming mails will not be available if there is no data connection. Existing messages are
usuallynotavailable offlineif there are nocache copiesdownloaded
- Large amounts of email and attachments may fill up and hit the space limit designated to the IMAP
account on the server
Internet Service Provider (ISP)
An Internet service provider (ISP) is an organization that provides services for accessing and
using the Internet. Internet service providers may be organized in various forms, such as
commercial, community-owned, non-profit, or otherwise privately owned. Internet services
typically provided by ISPs include Internet access, Internet transit, domain name
registration, web hosting, Usenet service, and colocation.
Classification:
1. Access providers ISP
ISPs provide Internet access, employing a range of technologies to connect users to their
network. Available technologies have ranged from computer modems with acoustic
couplers to telephone lines, to television cable (CATV), wireless Ethernet (wi-fi), and fiber
optics. For users and small businesses, traditional options include copper wires to provide
dial-up, DSL, typically asymmetric digital subscriber line (ADSL), cable modem or Integrated
Services Digital Network (ISDN) (typically basic rate interface). Using fiber-optics to end
users is called Fiber To The Home or similar names. For customers with more demanding
requirements (such as medium-to-large businesses, or other ISPs) can use higher-speed DSL
(such as single-pair high-speed digital subscriber line), Ethernet, metropolitan Ethernet,
gigabit Ethernet, Frame Relay, ISDN Primary Rate Interface, ATM (Asynchronous Transfer

5. Mode) and synchronous optical networking (SONET). Wireless access is another option,
including cellular and satellite Internet access.
2. Mailbox providers
A mailbox provider is an organization that provides services for hosting electronic mail
domains with access to storage for mail boxes. It provides email servers to send, receive,
accept, and store email for end users or other organizations. Many mailbox providers are
also access providers, while others are not (e.g., Yahoo! Mail, Outlook.com, Gmail, AOL Mail,
Po box). The definition given in RFC 6650 covers email hosting services, as well as the
relevant department of companies, universities, organizations, groups, and individuals that
manage their mail servers themselves. The task is typically accomplished by implementing
Simple Mail Transfer Protocol (SMTP) and possibly providing access to messages through
Internet Message Access Protocol (IMAP), the Post Office Protocol, Webmail, or a
proprietary protocol.
3. Hosting ISPs
Internet hosting services provide email, web-hosting, or online storage services. Other
services include virtual server, cloud services, or physical server operation.
4. Transit ISPs
Just as their customers pay them for Internet access, ISPs themselves pay upstream ISPs for
Internet access. An upstream ISP usually has a larger network than the contracting ISP or is
able to provide the contracting ISP with access to parts of the Internet the contracting ISP by
itself has no access to. In the simplest case, a single connection is established to an
upstream ISP and is used to transmit data to or from areas of the Internet beyond the home
network; this mode of interconnection is often cascaded multiple times until reaching a tier
1 carrier. In reality, the situation is often more complex. ISPs with more than one point of
presence (PoP) may have separate connections to an upstream ISP at multiple PoPs, or they
may be customers of multiple upstream ISPs and may have connections to each one of them
at one or more point of presence.[40] Transit ISPs provide large amounts of bandwidth for
connecting hosting ISPs and access ISPs.
Derivatives of ISP
1. Virtual ISPs
A virtual ISP (VISP) is an operation that purchases services from another ISP, sometimes
called a wholesale ISP in this context, which allow the VISP’s customers to access the
Internet using services and infrastructure owned and operated by the wholesale ISP. VISPs
resemble mobile virtual network operators and competitive local exchange carriers for voice
communications.
2. Free ISPs

6. Free ISPs are Internet service providers that provide service free of charge. Many free ISPs
display advertisements while the user is connected; like commercial television, in a sense
they are selling the user’s attention to the advertiser. Other free ISPs, sometimes called
freenets, are run on a nonprofit basis, usually with volunteer staff.
3. Wireless ISP
A wireless Internet service provider (WISP) is an Internet service provider with a network
based on wireless networking. Technology may include commonplace Wi-Fi wireless mesh
networking, or proprietary equipment designed to operate over open 900 MHz, 2.4 GHz,
4.9, 5.2, 5.4, 5.7, and 5.8 GHz bands or licensed frequencies such as 2.5 GHz (EBS/BRS),
3.65 GHz (NN) and in the UHF band (including the MMDS frequency band) and LMDS.
Transmission Media ( Bounded &
Unbounded)
Transmissionmedia isa pathwaythatcarriesthe information fromsendertoreceiver.We use
differenttypesof cablesorwavestotransmitdata. Data istransmittednormallythroughelectrical
or electromagneticsignals.Anelectricalsignal isinthe formof current.Anelectromagneticsignal is
seriesof electromagneticenergypulsesatvariousfrequencies.Thesesignalscanbe transmitted
throughcopperwires, optical fibers,atmosphere,waterandvacuum.DifferentMediashave
differentpropertieslike bandwidth,delay,costandease of installationandmaintenance.
Transmissionmediaisalsocalled Communicationchannel.
Types of Transmission Media
1. Wired or Guided Media or Bound Transmission Media
2. Wireless or Unguided Media or Unbound Transmission Media
1. Wired or Guided Media or Bound Transmission Media: Bound transmission media are
the cables that are tangible or have physical existence and are limited by the physical
geography. Popular bound transmission media in use are twisted pair cable, co-axial cable
and fiber optical cable. Each of them has its own characteristics like transmission speed,
effect of noise, physical appearance, cost etc.
2. Wireless or Unguided Media or Unbound Transmission Media: Unbound transmission
media are the ways of transmitting data without using any cables. These media are not
bounded by physical geography. This type of transmission is called Wireless communication.
Nowadays wireless communication is becoming popular. Wireless LANs are being installed
in office and college campuses. Microwave, Radio wave, Infra red are some of popular
unbound transmission media.

7. The data transmission capabilities of various Medias vary differently depending upon
various factors. These factors are:
1. Bandwidth. It refers to the data carrying capacity of a channel or medium. Higher
bandwidth communication channels support higher data rates.
2. Radiation. It refers to the leakage of signal from the medium due to undesirable electrical
characteristics of the medium.
3. Noise Absorption. It refers to the susceptibility of the media to external electrical noise
that can cause distortion of data signal.
4. Attenuation. It refers to loss of energy as signal propagates outwards. The amount of
energy lost depends on frequency. Radiations and physical characteristics of media
contribute to attenuation.