Every now and again, new technology throws something unimaginably irritating into the mix of modern life. People playing music through their mobile phones on the bus. Those self-scan checkouts in supermarkets that never recognise the last item in your trolley.

And, for those of us who use email: spam. No, we’re not talking about the canned luncheon meat, we’re talking about the emails you didn’t ask for – and don’t wish to receive – from companies and individuals offering discounted Viagra, touting get-rich-quick schemes or stock market scams, sending offensive messages and advertising pornography.

Spam is now so widespread that it accounted for an estimated four in every five emails sent worldwide at the end of 2006. Spam is up there with pop-up adverts on every web surfer’s list of most-hated things.

In this feature, we’ll explain why we receive spam and what you can do to stem the flow of junk into your inbox.

Why spam poses a danger
So how does spam find its way into your inbox? People who send spam – we’ll call them spammers – collect email addresses in different ways. One way is by good old-fashioned guesswork. This is more high-tech than it sounds, using specially developed software to generate likely email addresses by putting together known forenames and surnames, often using webmail domains, such as @hotmail.co.uk.

Using this method, the likelihood of sending emails to addresses that don’t exist is high, but as spammers don’t have to pay postage costs to distribute their unwelcome correspondence, there’s little to discourage this.

Another type of software beloved of spammers uses technology similar to that used by search engines. Instead of picking out keywords, it scours the web for email addresses posted in newsgroups and on websites. They may also buy email address lists from other spammers or from unscrupulous companies that misappropriate personal information users provide online when they sign up to a service. This is illegal in the UK, but proving where a spammer obtained an address is nigh-on impossible.

OK, so it’s annoying to have to delete a load of old tosh from your inbox every time you want to check your email, but is there anything really sinister about spam? Well, yes. Spam can contain and spread computer viruses. It may – and frequently does – contain offensive material and images. It can contribute to online fraud, including phishing scams, where an email that appears to have been sent from the recipient’s bank, for instance, asks them to ‘confirm’ their account number or email. We’ll look at how spammers can successfully assume the identity of a legitimate company via email in just a moment.

The tactics of spammers
To effectively identify and protect yourself from spam, it’s important to know why it’s sent and what its hallmarks are. Spam emails usually try to sell you something that usually sounds fairly improbable. Beware of emails from senders that you don’t recognise offering things such as discounted medication or cheap software.

Some spam messages appear to be complete gobbledegook and are often little more than strings of random words that convey no coherent message. This is because some spam is sent to check if your email address is real and in use. These typically contain a tiny image embedded somewhere in the body of the message that isn’t visible to the recipient.

The image is stored on the spammer’s website and when the message is opened or viewed in the preview pane of an email application, that email application visits the website to retrieve the image. This tells the spammer the message sent to your email address has been viewed and that the address is active. Once an email address is known to be real, it becomes valuable to the spammer, either to use or to sell on to other spammers.

It’s also possible to harvest email addresses from email chain letters. As well as emails promising luck if you send them to five friends within the hour, there are those promising a free iPod if you forward them to 10 people, as well as those warning of fictitious viruses and improbable personal safety warnings. As the forwarded email addresses become embedded in the email’s history, this will provide a bumper crop for the spammer behind it.