How Electronic Mail Works

Structure of an Email System

A Email system can be decomposed into several parts:

Message Transport Agents (MTA): The thing which shuffles message from one place to another. A network of MTAs can be called a Message Transport System (MTS).
Message Stores (MS): The place where a users Email is stored while waiting to be read by the user.
Mail User Agents (MUA): The program that a user runs to read their Mail.

Message Transport Agents (MTA)

The Simple Mail Transport Protocol (SMTP, RFC 821) and some other mail transport protocols, use the notion of store-and-forward. Mail messages are not necessarily sent direct from the sending user's machine direct to the final destination user's machine. Mail is forwarded from the sender's machine to a MTA that is "closer" (in some sense) to the receiver, to another MTA, ... until finally it arrives at its destination.

At each step along the way, the forwarding MTA passes on responsibility for delivering on the message to the next MTA. The forwarding MTA doesn't delete its copy of the message until the next MTA has acknowledged receipt of the message and taken responsibility for delivering the message. To guard against various failures, the next MTA shouldn't acknowledge receipt of the message until it has been written safely to non-volatile storage, i.e. disk.

Message Store (MS)

Sending and receiving Email isn't a synchronous event like a chat session that requires both the sender and recipient people to be online simultaneously for the message to be transferred. Normally there is some delay between when the sender sends the message and the recipient reads the message of some minutes, hours or even days.

It is also unusual for Email to be delivered directly to the user's desktop machine -- the user's machine is generally not running a mail receiving application all the time, or the user may use different desktop or portable machines at different times.

To cope with this, incoming Email is normally deposited into a message store, from which the recipient can retrieve their new messages to the workstation of their choice, at a time when it is convenient for them to do so.

In some cases, the user can also save their read messages back into other folders in the mail store. In other cases the, user will save their read messages in on their own local machine, or in a directory on a file server.

Mail User Agent (MUA)

There are several types of MUA:

Host-based MUA: The user has to log in to the multi-user host system (also acting as a Message Store) to read their Email. The MUA runs on the host system and manipulates the files containing the mail directly. Examples: MAIL on VMS.
LAN File server Email MUA: The user runs the MUA on their local desktop machine, accessing their mail stored on a file server. The MUA uses normal file manipulation operations to manipulate their mailbox. Read Email can be stored back on the file server or saved on the local machine. Examples: Pegasus Mail in LAN mode (DOS, Windows), Unix mail clients accessing mail over NFS.
Email Download MUA: The user runs the MUA on their local machine, which downloads their new mail from the MS to the local machine. If the user wants to keep the new mail, it must be stored on the local machine (or written to a network drive using a LAN file server protocol). Examples: Eudora, Pegasus Mail in POP mode (Windows).
Mail Protocol MUA: The user runs the MUA on their local machine, which speaks a mail-specific protocol to the MS. The MUA can access not only their new mail folder, but access and manipulate the contents of their other folders. Examples: PINE (VMS, Unix, DOS, Windows).