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)