To effectively identify and protect yourself from spam, it's important to know why it's sent and what its hallmarks are. Spam e-mails usually try to sell you something that usually sounds fairly improbable. Beware of e-mails 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 e-mail 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 e-mail application, that e-mail application visits the website to retrieve the image. This tells the spammer the message sent to your e-mail address has been viewed and that the address is active. Once an e¬mail 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 e-mail addresses from email chain letters. As well as e-mails 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 e-mail addresses become embedded in the e-mail's history, this will provide a bumper crop for the spammer behind it.