Spam is a modern plague, flooding inboxes with unsolicited "junk" email, promoting everything from dodgy get-rich-quick schemes, to Nigerian scams, to pornography sites. It costs businesses, ISPs, and you, the receiver.

It's a serious problem for millions of email users worldwide, and the stakes are high.

Why is spam such a problem?

There's the annoyance factor - sorting through a cluttered inbox looking for legitimate messages. There's the time wasted - important if everyone in an organisation spends 20 minutes or so a day dealing with spam. There's the chance of missing and accidentally deleting an important email.

ISPs and telecommunications companies also suffer, as they have to provide twice the bandwidth (throughput) to cope with the added load of spam. Extra staff have to be devoted to keeping up with spam issues, and dealing with irate customers.

Possibly worst of all is the prospect that out of frustration, people will turn away from the most efficient mass communication system ever devised.

Types of email

Because you can't always be certain whether an email falls into the category of spam, there are different "grades" of email.

Black email is undoubtedly spam - the obvious examples are get-rich-quick schemes, dodgy products, and pornography.

Grey email is less obvious, and depends, in part, on the attitude of the recipient. Imagine you dealt with a travel agency, and afterwards they kept bombarding you with special offers? For some that would be really annoying, but others might be happy to be kept informed.

White email is undoubtedly not spam.

Why do spammers do it?

You might think that most people would ignore the ridiculous offers and products - and you'd be right. But, spam costs very little to send. If a spammer sends out 100 million spams, and only 0.01 percent of recipients respond, that's still 10,000 sales.

Spammers make money. They are also very determined - even launching attacks to overwhelm and bring down anti-spam web sites.