As a general rule, every email that prompts you to "send a copy to everyone in your address book" (or similar text), is at best a nuisance hoax email; click here for an example, or here for a comprehensive list of hoaxes. If you have any doubts about the "warning" email you've been sent, check this list.

These hoax emails serve to clog up the ether: in June 2006 it was estimated that there were over 1.6bn spam emails circulating, but by November 2006 this had risen dramatically to over 7bn! What's more a considerable proportion of these are circulated by innocent users who react unthinking to what they've received and dutifully (!) pass them on.

They can also generate alarm or even mild panic. However there are also two further and far more serious consequences:

1. They might actually carry some malicious "payload" that you're then been duped into disseminating in a virus-like manner;

2. You are placing at extreme risk, every person to whom you sent the email. What's more some of the people in your addressee list may in turn then simply "forward" the email (unedited) to everyone in their address books; thus each email address in your original list now gets deposited in yet more strangers' computers and hugely magnifies the risks.

This second result is the real menace behind hoaxes like this, and here's how it works.

• Your computer may well be "clean" and have no viruses, trojans, spyware, and so on. But what about all the computers to whom you have sent the email? If just one of these has a virus, it will treat your email as a Xmas Dinner: every address embedded in the email will be harvested and used by the virus to spread itself yet further.

• Alternatively some nasty bit of spyware (on that one computer) would send itself to all sorts of other addresses around the world, using the addresses in your list as the apparent "senders"; known spyware emailers have their own email addresses blocked by most reputable email servers, but by using these third-party private email addresses, the spyware author effectively by-passes such constraints.

• A third way in which your list of email addresses could be used, is to simply add every address to a long list of "live" email addresses that is then sold on to other spammers, and in time every addressee in your list can expect to receive yet more offers to increase the size of organs many of them don't possess!

There are two very simple rules to follow:

1. Never, ever, forward emails of the type that prompt you to "send a copy to everyone in your address book". These may be warnings, jokes, charity supplications, whatever: you must assume that they are not what they seem.

2. If you must send an email to several people, use the "blind copy" feature: send the email to yourself as the prime addressee, then put everybody else in the Bcc field that most email facilities provide. In so doing, each recipient receives an email with just your email address (twice) and his or her own. All the others are *not* embedded in the email. Note that in Outlook Express, the "Bcc" option is not displayed until after you click on the "cc" button - don't ask me why!

See also Undelivered Emails on the left for further details of the consequences of forwarding hoax emails.