Protocols Behind E-mail
was the first business-to-business killer application
of the Internet. E-mail ranks with the printing
press, the telephone and television in mass impact.
Today, more than three billion-plus messages zip
back and forth each day and have become the oxygen
of the Internet.
is the preferred method for many personal communications
as well. A message may comprise from a few lines
to several pages. As an attachment, audio/video
clips and faxes can also be transported.
is the fastest, cheapest and reliable method of
transporting messages over long distances. Further,
unlike fax, the message is available right on your
desktop. The e-mail service is location independent.
A recipient who is away from his usual location
can connect to his e-mail server from anywhere in
the world to access new mail.
services were invented nearly 20 years ago. However,
the essential features of e-mail remain the same.
e-mail system is a distributed application. It deploys
TCP/IP protocol for reliable message transport.
typical e-mail system comprises message preparation,
'Message Transport System' (MTS) and the message
retrieval by the recipient. 'Simple Mail Transport
Protocol' (SMTP) was the basic protocol of the e-mail
system. It has the facility of message preparation,
transportation and retrieval. This protocol is the
workhorse of the e-mail system. SMTP is a simple
and reliable protocol of the TCP/IP protocol suit.
It was good for a homogeneous network. Now, the
Internet is a heterogeneous network and to overcome
this limitation, for message retrieval 'Post Office
Protocol 3' (POP3) was standardized. POP3 is a very
popular protocol and is widely used. However, it
supports offline working only. It does not support
message retrieval from multiple locations.
Message Access Protocol 4' (IMAP4) overcomes the
above limitations of the POP3 protocol. IMAP4 is
also an open standard. It has many useful features:
Offline, online and disconnected-user access modes
and message retrieval from multiple locations.
despite limitations, POP3 is widely used. Now, IMAP4
deployment is gaining momentum. A user is confronted
with a choice between POP3 and IMAP4 protocol.
Of E-mail System
The e-mail system is used to transport messages
between users over the Internet. It is based on
'Internet Engineering Task Force' (IETF) defined
open standards. It comprises of client-side and
serve-side parts. SMTP protocol governs server-to-server
and sending-client to mail server communications.
side: It is also known as the user side. It
is a human being (or a process on behalf of a human
being) wishing to use mail transfer service.
An originator or recipient of e-mail.
user: A user who originates a mail is termed
as the sending user.
user: The recipient of the mail is known as
a receiving user. A receiving user retrieves his
mail from the designated (also nearest) mail server.
recipients: A sending user may opt to send the
mail to more than one receiving users. These recipients
could be located anywhere on the Internet.
A sequence of ASCII characters of arbitrary
length that conforms to RFC 822 standard.
address: It comprises of two partslocal
part and domain name. The local part is the mailbox
ID. It is the name of an individual mailbox. The
domain name pertains to mail destination. It is
a hierarchically structured global character string
address, registered in the domain name server.
agent (UA): It is a client-side software that
is installed on a user machine. It performs dual
functions. While sending a mail, it functions as
sending UA and while retrieving the mail, it functions
as receiving UA.
UA: It accepts the message and formats it as
per RFC 822 standard. The sending UA adds header
field that comprises e-mail addresses of sending
user and receiving user, date and subject field
is also responsible for establishing TCP connection
with the nearest (designated) mail server and sends
the formatted mail. It follows SMTP defined rules
for transporting the mail.
also acts as SMTP client to sending mail server.
UA: It is installed on receiving-user's machine.
It establishes TCP connection with the designated
receiving mail server to retrieve the mail. It uses
one of two standard protocols (POP3/IMAP4) to retrieve
side: It is also known as 'Mail Transport System'
(MTS). It comprises of a network of mail servers
over the Internet. Each mail server functions as
'Mail Transfer Agent' (MTA). MTAs mediate mail server-to-server
communication using SMTP. A MTA is installed on
every mail server. It can function as a sending
MTA, receiving MTA and an intermediate MTA.
MTA: It is the designated MTA (mail server)
for the sending user. It performs the following
accepts mail from the sending UA using SMTP.
obtains the IP address of the recipient from
domain name server, using given e-mail address.
tries to transport the mail directly to receiving
MTA, if possible. Alternatively, it obtains
the IP address of an intermediate MTA and relays
the mail using SMTP.
MTA: It is the designated MTA for the receiving
user. It is installed on a receiving mail server.
It is responsible for reception and storage of mail
in the mailbox of the receiving user.
a receiving mail server along with MTA, 'Mail Delivery
Agent' (MDA) is also located. MDA shares local work
with MTA. A receiving UA contacts MDA for its mail.
MTA: For end-to-end transport of e-mail, SMTP
compliant intermediate MTAs are installed at the
network nodes. An intermediate MTA relays/forwards
the mail for onward transport towards receiving
The receiving mail server stores the incoming mail
in the mailbox. It is the designated space in memory
for the receiving user. It is identified by a unique
ASCII character string.
retrieval: A receiving UA can retrieve (access)
mail in offline, online and disconnected-user access
modes. Here, is a brief description.
access: Here, a client application (receiving
UA) periodically connects to the receiving mail
server, either when a user dials in or at some preset
interval. The receiving UA downloads all messages.
are deleted from the receiving mail server's mailbox.
Now, messages can be processed at the user end.
Offline upload: Similar to offline access, messages
from the sending end can be transferred using client
access: Here, connection between the receiving
end user and the receiving mail server is maintained
all the time. The receiving UA does not store the
messages locally. It only retrieves the messages
from the mailbox as needed. Here, connectivity between
mail server and receiving user is essential for
access mode: This access method has the characteristics
of both offline and online access modes. Here, a
user can connect to mail server periodically to
download the messages. These can then be read, deleted
and organized in offline manner.
while messages are downloaded to receiving-user,
the same are not deleted at the server end. Thus,
when a receiving user again contacts to the mail
server, he can access previous mail as well. This
non-deleting feature for mail enables a user to
read his mail from multiple locations as well.
format standard: E-mail system uses RFC 822
for formatting an e-mail. This protocol is defined
for text messages. The SMTP protocol requires that
RFC 822 is used for constructing e-mail. This protocol
considers message as comprising of two parts: envelop
envelope contains information fields, which are
required for transporting and delivery of a message.
The envelop contains arguments pertaining to date,
from, subject, to and cc etc.
envelope and message body are separated (delimited)
by a blank line. Message begins with 'Hello'. Message
is in the form of memo and comprises of ASCII characters
It is an application level protocol. It is a member
of TCP/IP protocol suit. It deploys two-way (duplex)
communication channel for transporting/relaying
messages. It requires a reliable transport protocol
such as TCP for reliable delivery of mail. SMTP
jips/relays messages between two mail servers or
between sending UA and the sending mail server.
SMTP is independent of message format/content. However,
for standardization of the e-mail system, it requires
the use of RFC 822 for formatting/constructing e-mail.
requires use of a 7-bit ASCII character set. However,
TCP protocol supports 8-bit characters only. Thus,
higher-order bit is set as clear zero to accommodate
tags information regarding the route followed by
a message in the header portion. SMTP uses TCP for
message transport. However, it does not guarantee
to recover from the lost messages. Here, no end-to-end
acknowledgement to the sending user is sent for
a successfully transmitted message. Further, error
indications are also not returned.
cannot transmit executable files. For transmission
of binary files using SMTP, it is necessary
to convert binary files into text files. There
are many popular methods such as Unix UU encode/UU
decode and Bin Hex. However, these are not standard
cannot transmit text data that contains national
language characters. These national language
characters use 8-bit codes with values of 128
decimal or more.
is limited to 7-bit ASCII characters only.
servers may reject mail messages beyond some
gateways to X-400 electronic mail networks cannot
handle non-textual data including X.400 messages.
Some SMTP implementations do not adhere to standards
completely. Some of the typical problems are:
addition or recording of carriage returns and
or wrapping lines longer than 76 characters.
of trailing white space.
of lines in a message to the same length.
Internet Mail Extension' (MIME): MIME attempts to
resolve the above limitations of SMTP. It extends
the RFC 822.
includes the following elements.
new message header fields are defined. These
include MIME version, content type, content
transfer encoding, content ID and content description.
These fields provide the information about the
body of a message.
number of content formats are defined. These
enable the e-mail system to support multimedia
transport as e-mail attachment.
encoding standards have also been defined. These
enable conversion of contents from one format
to another format. Thus, now contents cannot
be modified by the e-mail system during transport.
It is the most common client protocol used for retrieving
e-mail from the receiving mail server over the LAN
or the Internet. It enables intermittent checking
for messages stored at the receiving-mail server.
important features are:
As default mode of operation, it supports offline
access mode. A POP3 compliant mail server enables
a receiving UA to download the mail. After successful
download, the mail is deleted at the mail server.
POP3 compliant server can also be asked to retain
the message after downloading. However, in this
mode, when a receiving user establishes a new connection,
it will download all the previously stores messages
along with attachments, again and again.
of the limitations are:
the storage space in the mailbox is limited
and POP3 server should inform about impending
exhausting of quota. POP3 has no provision for
sharing mailboxes or messages.
has limited capability to handle complex messages
that contain multimedia attachments.
client program is the best suited for people
who have one mailbox and read their e-mails
from one PC.
does not have the facility to encrypt user names
Mail Access Protocol' (IMAP4): It is a newer e-mail
retrieval protocol. It is described in RFC 2060.
It has not been implemented widely.
salient features include:
requires a reliable and ordered data stream
such as provided by TCP. When TCP is used, IMAP
compliant mail server listens on port 143.
supports online, offline and disconnected-user
access modes. This allows a client to access
and manipulate e-mail messages on a mail server.
permits manipulation of remote mailboxes like
a local mailbox. This includes operations for
creating, deleting and renaming mailboxes, checking
for new messages, permanently removing messages,
setting and clearing flags etc.
clients do not send the complete contents of
every file. The IMAP mail server sends a short
menu of waiting messages. Thus, important messages
can be transported quickly.
messages are stored on mail servers only. Thus,
messages can be accessed from multiple IMAP
clients and a user can still see the same status
information for all messages.
server understands MIME file extensions. These
enable an IMAP client to select the desired
portion of an
e-mail for retrieval.
allows the messages to be stored in a hierarchical
structure on the IMAP server.
sends the passwords and user names as encrypted.
Contrary to offline access mode supported by POP3,
IMAP4 supports offline, online and disconnected-user
allows users to access and manage messages from
more than one location/ computer.
supports folder hierarchies and concurrent access
to shared mailboxes.
has database capabilities. This enables a user
to search and select messages and part of messages
stored on the server.
supports MIME. Thus, a user can access audio/video/graphic
does not rely on file-access protocols and does
not need to know the server's file storage format.
Changing a mail system from POP3 to IMAP4 is easy
but reverting back to POP3 and can be an administrative
nightmare. Compared to POP3, IMAP4 server requires
more storage. Thus, storage could be critical. Compared
to POP3, server connect time to IMAP4 server could
be longer. Users may need time to review message
headers and deciding which attachment he likes to
download. IMAP4 server could lead to higher technical
support cost. Thus, it may be best suitable for
controlled environments i.e. universities and large
corporates. IMAP4 by itself offers no solution for
calendaring or scheduling. One has to adopt other
access services: E-mail system has global nature.
There exist several e-mail systems over the Internet.
of these e-mail systems have their own directory
services that give information about users, systems
and services. Standard access to these different
directories is possible by using X.500 and 'Light
Weight Directory Access Protocol' (LDAP).
X.500 is a directory assistance system for computers
in a distributed network. Such a directory contains
e-mail addresses of all users. X.500 directory services
define different object classes that can be used
to identify objects within the directory service
tree. These objects include aliases, country codes,
localities, organization, organization units and
people. However, implementation of such a global
directory requires vast storage and computational
The LDAP is designed to provide access to X.500
and non-X.500 directories. It is standardized by
IETF (RFC 2251). LDAP is specifically targeted at
management applications and browser applications
that provide read/write interactive access to directories.
LDAP is designed to run over connection-oriented
reliable protocol such as TCP.
uses 8-bit coding for a character.
standard: It is an international e-mail interchange
standard. It is standardized by ITU-T. It is complex
and costly to implement. Thus, its use is confined
to large enterprise-wide networks and commercial
e-mail service providers. It encompasses all electronic
messaging technologies such as telex, tele-text,
and facsimile and proprietary e-mail systems.
The latest trend in e-mail systems is towards using
open standards. Thus, SMTP is the widely used protocol
for relaying mail over the Internet. As regards
mail-retrieval by the recipient, there is a choice
between POP3 and IMAP4. Here, is a brief description
of the available IMAP4 implementations.
Path offers 'In Scribe Messaging Server' and 'InJoin
Directory Server'. These run on Windows NT/2000,
Solaris and SGI Irix and are claimed to scale up
to million users.
offers 'World-Mail Server 2.0' for Windows NT/2000.
It is based on Isocor N-Plex code. It is easier
for small organizations to install and run it.
offers 'NTMail'. It can be deployed on Windows NT/2000,
Solaris and Linux. It supports upto 10,000 users.
It has been selected as 'Editor's Choice' by PC
Magazine and Network Computing reviews. It supports
'Wireless Access Protocol' (WAP) also.
Technology offers 'InterChange'. It combines an
e-mail server, HTTP server and e-mail client proxy
into a single package. It can be used as a standalone
server also. It provides IMAP, POP and SMTP services.
Alternatively, it can provide web-based gateway
to e-mail systems that do not offer one on their
own. It can support WAP also.
offers several different IMAP-compliant messaging
servers. The offering includes the 'Sun Internet
Mail Server' and 'Netscape Messaging Server'. Sun
Internet Mail Server requires the Solaris operating
system. Netscape Messaging Server runs on Alpha,
PA-RISC, Linux and NT. Both servers are carrier
offers 'Mail Server' for NT/2000. It costs US$1500
for unlimited use and presupposes modest system
offers standards-compliant software into server
appliances that supports 300 to 2500 users depending
on the model. It is claimed that it works fast with
WAP since they do not need gateways to translate
between the WAP and IMAP
offers 'Novell Internet Messaging' system. It can
run on NetWare, Solaris and Linux.
system' offers 'Mailsite' that runs on Windows NT/2000.
It can be used for upto 1 million users per server
is a well-known freeware program. Now, it has been
enhanced and supported commercially. It is claimed
that it can support hundreds of thousand users on
a single server. It also supports IMAP folder management
Domain Naming System (DNS): DNS is a member
of TCP/IP protocol suit that resolves IP addresses
Message Access Protocol (IMAP):
IMAP is a member of the TCP/IP protocol suit that
enables a client to retrieve the mail from the mail
server. RFC 2060 standard describes this protocol.
It can operate in online, offline and disconnected-user
access modes. It allows messages to be stored in
a hierarchical order on the IMAP server.
Office Protocol (POP): It is a member of the
TCP/IP protocol suit. It is used by a recipient's
user agent to retrieve mails from the mail server.
Its latest version is POP3 and is described in RFC
Mail Transfer Protocol (SMTP):
It is a TCP/IP standard for mail interchange between
mail servers and between mail server and sending
user. It is described in RFC 821.
It is an e-mail address that actually refers a different
e-mail mailbox. Aliases are commonly set up to reference
a function within an organization.
Internet Mail Extensions (MIME): It is a file
encoding method that provides a mechanism for translating
non-ASCII messages into ASCII format for transmission
over the Internet. Different MIME types are defined
that allow each file category to be encoded in a
defined manner. It is an extension to RFC 822. It
overcomes the limitations of SMTP. The specification
is provided in RFC 1521 and 1522.
822: It defines the format for an e-mail. Here,
message is viewed as having envelops and contents.
Bin Hex: It is a storage protocol. It translates
a binary data file into an encrypted text version
UU encode/UU decode: Unix to Unix encoding is an
alternative method for converting raw binary data
into a text representation. It is used for sending
binary attachments via a text-based Internet mail
is GM (R&D) ITI Ltd. Naini, Allahabad. He can
be reached at email@example.com