The phrase 'killer application' is bandied about far too loosely. In truth, you can probably count the number of computer technologies that have genuinely changed the world on the fingers of one hand, and email will almost certainly account for one of those pinkies.

Now 32 years old, email is nothing short of a phenomenon. An estimated 50 billion messages are sent every day, the volume doubling roughly every 18 months. Part of the reason for its widespread popularity is its simplicity. People who struggle to come to terms with Windows, and who glaze over at the very thought of spreadsheets and instant messengers, still know and understand email. Even the Queen sends them.

Businesses are fond of email for two different reasons. First, it establishes a two-way link between them and their customers, without third parties getting in the way. And second, it's a lot cheaper and quicker than sending 'real' mail or making telephone calls.

But it works for their customers, too. Email has brought a new age of commercial openness which, in the long run, could help to bring down prices and deliver more relevant products to everyone.

At home, email makes it easier to keep in touch with friends and relatives. We can zap a message to a loved one half-way around the world in a matter of minutes, and even attach a photo or a useful document, or share our greeting with several recipients at once.

What is email?
Email started out in the early 1970s as a means of leaving digital notes between users of the same mainframe. It was an MIT graduate named Ray Tomlinson who worked out the means of sending those notes between computers and, eventually, across the internet.

Basically, email works like this: when you send an email message, your email client (Outlook Express, for example) connects to your ISP's mail server. This is typically a Simple Mail Transfer Protocol (SMTP) server, which then checks the address against an online registry (or DNS server) to find the recipient's unique Information Protocol (IP) address. This whole process takes a matter of seconds and, once completed, the email is sent on its way.

How does it arrive at its destination?
Email travels in an interesting way. Rather than being transported as a complete file or document, it is broken down into smaller packets of data, with each one finding the quickest route to its destination. Imagine it like the classic car chase from The Italian Job, each Mini bearing part of your message around a congested digital traffic system.

At its destination, email is usually parked in a POP server, or shunted between other servers until it reaches the right one. What follows is the slowest part of the whole procedure, because everything now stops until the user logs on and retrieves the messages that are waiting on the appropriate server, at which point the parking space is cleared for the next delivery.

What does the address mean?
The familiar email address structure is crucial for getting your message to its destination, and must be letter and case perfect. Typically it has a username, the @ locator symbol and a domain name, which may be split into several parts. So typing in joe@sales.bloggs.co.uk ensures that the email is transported straight from your PC to the right inbox of the right department.

Given that this incredible delivery system is often provided free of charge by your ISP, it's no surprise email became the fastest-growing medium of the 20th century.

What made it so popular?
Email's success was, in part, down to factors such as speed and cost. In the late 1970s, faxes were generally considered to be fast enough for most needs, but they had to be sent individually and became more expensive the further you sent them.

The beauty of email was that hundreds (and later millions) could be dispatched simultaneously to anywhere in the world at the cost of a single local phone call. And yet not everyone is happy. There are those who argue that society is sacrificing a great deal for the sake of speed.

When we talk about the problem of information overload, we're often referring to email abuse. People copy each other in on memos unnecessarily, CC (carbon copy) each other on jokes, forward yet another unfunny viral JPEG to another ungrateful list of contacts.

On average, employees spend 20 minutes wading through their emails every morning and, as the volume keeps rising, so does the cost of dealing with them. In a recent survey for direct marketers Harte-Hanks, 31 per cent of respondents said they were already receiving too many emails at work, and 48 per cent at home. And then there is spam ...

Email's Achilles' heel
If email is software's equivalent of Superman, then spam is Kryptonite; not just a colossal pain in the neck for computer users the world over, but now such an epidemic that it threatens to kill the killer app itself.

Spam now accounts for between 50 and 70 per cent of all emails sent, depending on which country you live in. Arriving on the scene not far behind the enthusiastic consumers and businesses who first took up email, spammers were quick to exploit the email platform for their own gains.

It wasn't long before they learned how to harvest email addresses from newsgroups, websites and unscrupulous e-commerce sites. Today, anti-spam consultancy Spamhaus claims to detect up to nine billion spams a day, many of which carry a viral or pornographic payload.

Fortunately, the fight-back has begun in earnest. In March this year, 10 major ISPs (including AOL, Microsoft and Yahoo) united to file lawsuits against hundreds of spammers, using America's sweeping new CAN-SPAM legislation.

One of the defendants was accused of sending out 100 million spam emails in January alone, adding weight to the widely held belief that around 200 dedicated spammers are responsible for 90 per cent of the global problem.

With more vigilant ISPs and a more secure version of Windows (codenamed Longhorn) in the wings, Bill Gates is not alone in believing that spam can be beaten in two years. Others, however, are not so optimistic.

Sendmail, which claims that its software is behind the majority of emails, says that even if ISPs filtered out the majority of spam, users would still be deluged by it.

"Spam is doubling in Europe every eight to 12 weeks," said Sendmail's vice president, Greg Olson. "And with that kind of curve it isn't long before 95 per cent just isn't good enough, because we'll still be getting 100 spams in our inboxes every day."

Negative effects
The rise of both spam and viruses corresponds exactly to the growing popularity of email, and fighting those two alone costs the business world billions per year. There are also peripheral effects, not least existing industries being forced into premature decline.

The Royal Mail's troubles may be down to many factors, but it certainly doesn't help seeing banks, utility companies and supermarkets switching bill payments and statements to email, or indeed hearing your flagship service referred to as 'snail mail'.

You could, however, throw the same argument at every major innovation of the past 300 years. The steam train spelled the end for the carthorse, and email may or may not eventually kill off cheap postal mail in the same way.

But this doesn't mean that email's dominance is assured. And if spam or viruses don't get it, it could yet be undone by the very trend it set in motion. In the face of fast-expanding mobile phone technology, email is increasingly becoming one of the slower forms of digital communication.

Instant messaging and SMS text messaging are both significantly faster, taking seconds rather than minutes to reach their destination. So, although email usage is predicted to grow quickly for the next 10 to 15 years at least, it could then tail off as the majority switch to always-on or mobile-based messaging systems.

Fast forward
That version of the future is a long way off, however, and email is still a relatively new kid on the block. In its short lifetime, email has already firmly entrenched itself in the world of business, and is so widely used that it even has its own code of conduct (see Netiquette below).

If nothing else, the speed and convenience of email, combined with its proven adaptability and relatively low cost, should ensure its longevity. The threat of spam and viruses is certainly not something to sniff at, but judicious use of the right preventative measures - an up-to-date anti-virus program, a firewall and a junk mail filter - will help to keep you, your family and your PC safe from anyone who abuses the email platform for malicious reasons.

Netiquette
Despite being relatively young, email has quickly developed its very own conventions and formalities. Netiquette, as it is known, began life merely as a question of consideration for others, for example, not overusing CAPITAL LETTERS or complicated fonts.

These days, however, when whole networks can be brought down by a single worm, the unspoken rules of email are more important than ever, and proficient users will not hesitate to add you to their junk senders list if you ignore or abuse them - even inadvertently. Below are some of the most crucial netiquette rules for email users:

1. If you don't have anti-virus software, don't send email attachments. In fact, try not to send bulky attachments at all; they take ages to download and some ISPs restrict or block them altogether.

2. Don't automatically pass on chain letters, surveys or questionnaires to everyone in your address book, no matter how heart-warming, scary or amusing you think they are. Not everyone you forward these to will thank you for them.

3. Don't click the 'Reply to All' button unless you really need to. It's bad enough to be included on a long list of recipients without having to hear all their lame replies too.

4. Don't assume that email is instant. Equally, demanding 'read receipts' and putting 'important' tags on everything you send can often cause annoyance to those on the receiving end.

5. Don't 'flame mail'. Even if someone has made you really cross, a calm, measured response is better than an angry, impetuous message - and you can't take it back once you have hit 'send'.

6. Be courteous. Email may not be as formal as letter writing, but that's no excuse to be sloppy, rude or impolite. Respect the privacy of friends and strangers and you'll be fine.

Ray Tomlinson
In 1971 Ray Tomlinson was working as a programmer for the consulting firm of Bolt, Beranek and Newman (BBN), on an Arpanet time-sharing project called Tenex. His big idea was to combine two existing protocols - one called SNDMSG for leaving messages for users of the same computer, the other Cpynet, for transferring files across the Arpanet.

He later recalled that combining these two "probably took four, five, six hours to do. Less than a day spread over a week or two when I had a spare moment." Perhaps unfairly, Tomlinson is best remembered for deciding to use the @ symbol in the address as a means of distinguishing between local and network addresses.

Not that he helped his cause - he kept no formal records of his bigger contributions, for example the content of those historic first emails. "It never seemed big at the beginning because there weren't many computers," he explained. "We didn't call it email. If we called it anything we called it mail or messages."

Email went public in early 1972, and within 12 months accounted for 75 per cent of all Arpanet traffic. Trillions of emails later, Tomlinson remains in two minds about exactly how much he changed.

"I don't think people are fundamentally different now than they would have been. They simply communicate more," he said. "Maybe they've made friends and maintain relationships that they wouldn't have. But bad guys are still bad guys. Good guys are still good guys. Friendly people are still friendly. People have found answers to questions and email has been part of that solution."

Now 64, Tomlinson remains at BBN where he is currently working on distributed network applications. So, how does he view the type of cult fame he now enjoys? "It's a geek thing," he said.

"Computer nerds know that I've done this. I've got emails from individuals who've run across this fact. They say 'It's great what you did. Why don't you do something about spam?' It's not the centre of my life."