E-mail is either the saviour of modern society, and trees, or the devil on the icing on the cake of technocracy. Electronic mail, at its simplest, is a replacement for paper letters (``snail'' mail) or the facsimile/fax. Sending email is easy, if you know the address of the person you want to get it to. You type in the message using whatever facility you are familiar with, and then submit it to the mail system (put it in the postmasters postbag!). Then a series of automatic systems (message handlers) will sort and carry it to the destination, just like post offices and sorting offices do with paper mail.
Here is a message one of us got recently illustrated in table 2.2.
Table 2.2: Example of Electronic Mail
The protocol used for electronic mail in the Internet is called the Simple Mail Transfer Protocol or SMTP. The model is of Message Handling Systems and User agents all talking to each other. Both use same protocol. User program invokes SMTP to send to a receiver.
The receiver may be a mail relay or actual recipient system.
SMTP Mail address look like this: J.Crowcroft@cs.ucl.ac.uk. The general form of such an address is:
User @ Domain
The Domain is as defined in the Domain Name service for host implementing SMTP. The DNS Name is translated to an IP address. The sending system merely opens a TCP connection to the site and then talks the SMTP protocol.
SMTP control is ASCII (text) based commands and responses over the TCP connection:
After the sender connects, commands include:
Table 2.3: The Simple Message Transfer Protocol
You can type this!
Message ``Body'' includes fields like inter-office memo:
Table 2.4: Message Heading Fields
The site you connect to may be a relay. Relays are configured by hand, with routing tables, in a way exactly analogous to Post Office Sorting Offices. Systems often use default entries to mail out of large site.
The receiving system may be an info-server. This is a special mailsystem that delivers messages to a server program rather than a human or mailbox. This program will understand some simple set of commands in the message. Often these are a subset of File Access Commands from FTP. Thus a user need only be familiar with E-Mail rather than the whole gamut of possible obscure commands to an FTP system to be able to send or retrieve documents from a server.
The SMTP protocol that carries electronic mail in the Internet is restricted to 7 bit clean ASCII (i.e. printable characters only). Also the body of an RFC822 complaint message has no structure.
Both of these shortcomings are removed by the MIME standard, which is described in the next chapter along with WWW in some detail.
A third shortcoming of Internet mail is the lack of security mechanisms (e.g. Privacy, Authenticity, Integrity, Non-Repudiation etc) that are required if we wish to use it in a legal (say contractual) basis. These are fixed in the Privacy Enhanced Mail standard. However, export controls between some countries, and legal restrictions on private or corporate use of encryption in others are hindering the deployment of this technology.