SMTP

SMTP manages the transfer of e-mail from one computer mail system to another.
It does not accept mail from local users nor does it distribute mail to the intended recipients. This task is handled by the local mail system.
SMTP only interacts with the local mail system and thus does not see the mail which is local to the system. It is only when mails are sent to or received from another machine does SMTP come into play. An I/O queue exists as the interface between the Local Mail System and the Client/Server ports. The client is concerned with sending mails to another system while the server is concerned with only receiving mails.
The local system thus maintains a mailbox for each user on the system.
The name of this mailbox is unique and consists of two ports:
The Local Port
This is just the name of the user and must be unique to the local host.
The Global Port
This part is the name of the host and must be unique to the internet.


MHS
This is used in Novell messaging products.
Similar to X.400 the MHS server acts as a gateway and translates the format of a message if required.

X.400 addresses

X.400 email addresses have many possible entries mostly optional.
The values generally used are:
Country
the country must be specified
ADMD
the identity of the X.400 service provider
PRMD
the top level of the organization using X.400
Organization and organization units can be used to subdivide the company into logical groupings. The user is identified using some combination of first name last name initials and nickname.

X.500 Directory Standard
X.500 is a standard that is applied to directory services rather than email systems.
An email system can implement X.500 within its recipient directory and can then integrate it with other X.500 compliant directories.
X.500 directories can be organized into a single global directory than can offer thousands of names for querying on the Internet.
X.500 allows data to be organized in a hierarchical system in which users can be located by organization department or name.
One of the most popular X.500 directory services is Novell NetWare Directory Services (NDS).

X.400 Protocol

X.400 was defined by the CCITT (a French acronym for International Telegraph and Telephone Consultative Committee).
This committee is now named ITU-T.
X.400 is the universal protocol for e-mail. It defines the envelope for e-mail messages so all messages match to a standard format.The CCITT recommendations X.400 through X.430 define an Application layer protocol and a minimal Presentation layer protocol.
CCITT X.400 uses the ISO Session Layer services and protocol documented in ISO documents
X.400 comprises three major components:
The User Agent (UA)
The User Agent has immediate interaction with the system.
The user agent allows the user to compose submit and receive mail messages.
The messages that the user receives are not just Inter Personal Messages (IPM) but the user can also receive receipt (and non-receipt) notifications and delivery reports.
The Message Transfer Agent (MTA)
The MTA acts as the message store and the forwarding agent and can also act as the gateway by translating messages into the correct format for the user.
Thus MTA is where the actual transmission of messages take place.
They also perform all the needed routing and delivery functions.
In order to avoid spurious non-delivery problems for the UA the UA had to be available from the MTA at all times.
This problem was solved with the introduction of Message Store (MS).
It is MS which is available to the MTA at all times and supplies a store and forward service to the UA.
This is particularly useful in the more distributed modern environment where the User Agent may be running on a users personal machine and hence might not be available for long periods of time.
The Message Transfer System (MTS)
This provides the messaging infrastructure through which users can communicate.
The MTS comprises a group of MTAs.
The Access Unit (AU)
For the Inter Personnel Messaging Users to communicate with users of pre-X.400 technologies such as telex teletex and even postal delivery services the AU's were introduced. An AU is a device that acts like an interface between MTS and pre-X.400 on the other side. It converts formats service elements and protocols and provides non-X.400 services bi-directional flow of messages.

Email Standards

Email standards ensure that different networks using different email messaging products can communicate with each other reliably and consistently.
The three most widely used email standards are:

X.400

Simple Mail Transfer Protocol (SMTP)

Message Handling Service (MHS)

E-mail

One of the most common network applications is electronic mail or e-mail.
E-mail is fundamentally different from paper-based communication.
Since the turnaround time is fast email is more conversational than traditional paper-based media.E-mail messaging systems use the 'store and forward' principle to send and receive messages. The server acts as a central repository for messages i.e. if a user who is not currently connected to the network receives a message it is stored at the server until the user connects and downloads the messages.
Components of an E-mail system The components of an e-mail system are as follows:
The User Agent/Email Client
This performs tasks such as composing mail maintaining address books notifying users when an email arrives and sorting mail after it arrives in the Inbox.
The Message Store/Post Office
This holds the mailbox for each user and ensures that incoming messages are passed to the correct recipient.
The Message Transfer Agent (MTA)
This is responsible for forwarding messages when they are destined for a recipient located on another Post office.
The Access Unit/Gateway
This is used to send messages mail systems that use software and standards different from the one you are using e.g. LAN based messaging systems that communicate with Internet mail servers.
gateway must be able to physically route messages to their intended destination message store and must also translate the message content and reply address to a format understood by the destination system.
There are various levels of sophistication in a messaging system.
Some provide advanced features such as delivery receipt and encryption of messages.
Some of the widely used LAN based messaging systems are:
Microsoft Exchange
Lotus CC Mail and Notes
GroupWise

Network Applications

Local Area Networks were initially designed to support only shared disk resources and peripherals.Even though software can make use of network services for file access and printing they were essentially built as 'stand-alone' applications.Since then many applications have evolved which are designed to enable users to work together using the advantages of the facilities offered by the network. Some of the network applications include e-mail scheduling groupware and shared applications

Creating and Managing Accounts

Every network operating systems have built-in applications which allow the administrator to manage user and group accounts.
For example Windows NT has User Manager for Domains while NetWare has NWADMIN / NETADMIN.
The user needs to have administrative privileges before it can manage accounts on the system.

Default User Accounts
Most of the network systems have default user accounts that are created at the time of installation of the operating system.
Along with the user account and administrative account in some cases the guest group is also installed to provide minimum access to the resources for occasional users.

Administrative / Supervisory Account

Administrative account which had access to all services and resources on a server is available by default on all server based network operating systems.

These accounts are normally used when any security or configuration related task has to be performed on the server.

Administrator can also create other users with rights which are similar to that of an administrator.

Note: It is recommended that limited number of administrator accounts should be made as they are potential security loopholes.

User Level Security , Server Based Security

User Level Security
It is normally used on server-based networks where the administrator shares resources like folders and printers and makes them available for network users.
The resources can be protected by using high security option where each user has to logon to the system and have the rights to use the resources.
This type of security is available in Windows NT, Netware and Unix based systems.

Server Based Security
The user needs to supply a username and password to log on to the network which is then compared with the servers security database to authenticate the user
The username and group membership of an authenticated user is stored in an access key generated by the server security service or S3.
All resources on server-based systems have an access list which contain entries for all users and groups.
These entries have permission to use the resource thus controlling the access to resources.
The access list also contains the level of access available for each entry.
Whenever a user attempts to access a resource S3 matches the users access key with the entries in the list to decide if the user has permission on a resource and up to what level.

C2 security
C2 grade has been made mandatory by the US Government for all network operating systems to ensure at least some amount of security in computers.
The C2 grade has been decided by the National Computer Security Center which also defines other security levels and features.
Microsoft Windows NT and Novell NetWare 4.x have got C2 grade certification from the US Government.

Network Operating System Security

Network operating system use one of the two security models:

Share level security

User level security

Share Level Security

It is often used on peer-to-peer networks where each user can decide which resources of the machine should be shared among other users on the network.

The shared resources can be protected from other users via passwords as a mean of authenticating the user requesting the resource.

For example A windows 9x machine can provide the following access privileges when sharing a folder:

Read-only access based on passwords or without a password.

Full access based on a password or without it.

Choice between both access levels depending upon the password entered by the user requesting the resource.

The owner can also secure resources like printers with the help of a password.