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Basic Email Requirements

Having been made the subject of a hit Hollywood movie, You’ve Got Mail, email has clearly made the transition from technology culture to popular culture. We all know what email is, pretty much. Here’s my definition: Email represents all the systems and mechanisms by which a message entered into a network-connected device finds its way to a destination device. The way we normally speak about email encompasses the messages themselves, the systems that handle the delivery of the messages, the software that allows users to send and receive the email, the specifications that define how those messages are formatted, addressed, sent, transmitted, and received. You’ve mastered email if you can understand how those five things— formatting, addressing, sending, transmitting, and receiving—work. Those five things are what the standards are all about, and what this book is all about. Those things work in specific ways for Internet email, but they don’t have to work that way for all email systems. As far as the user is concerned, email means a piece of client software that somehow sends and receives messages. Through the use of that software, it is possible to enter a message and a destination address to whom the message should be delivered. After the sender sends the message, it appears in the destination user’s mailbox. The mailbox refers to the part of the client software that displays email messages, and it is also the part of the client software that allows the user to access and read messages that have been received.
This is the way most end users experience email, whether it is proprietary email like Lotus’s cc:Mail or open-standard Internet email. There can be significant differences in what happens to the email after the sender clicks the send button and the recipient opens the message. Traditional proprietary email systems were based on single systems with many users, so they were relatively easy to build. You just had to set up a message storage system and an application that would notify recipients when they received a new message. Users log on and are given access to read messages they received (or sent to others). The messages are all stored in a central repository. There is no need for networking beyond what is necessary to connect users to that repository. Messages never leave the central system.
This approach to email has many advantages. It is simple to build and deploy. There is no need for complicated networking tasks relating to email. A mechanism for users to send and receive messages is required, but this can even be provided through a simple application built on the email server itself and accessed through a terminal session. An administrator can handle message backup for all users. Messages can be delivered instantaneously.
However, the central server model has its drawbacks as well. The entire system has a single point of failure and when the server goes down, users have no access to any of their messages. Messages to recipients not in the system must be handled through other means. Unless old messages are expired and removed, central email systems can quickly fill as much disk space as you throw at them. Message retrieval performance can degrade as the message store increases in size.