We have all done it, of course. A slip of the keyboard changes [email protected] to [email protected] and an urgent e-mail is bounced back because the address does not exist among the millions of internet domains that identify us all online.

At least when you get an e-mail address seriously wrong you find out about it, because the server computers that deal with the billions of messages sent every day are set up to report errors back to the sender.

It happens because when you send an e-mail to someone it goes the nearest mail server - the one in your organisation, or one provided by your internet service provider (ISP). It is then delivered to a computer identified by the last part of the e-mail address - the bit after the @ symbol.

If your mail server can not find that computer then it looks to see the reason why. If it is just that the computer it is trying to send a message to is not responding, perhaps because it is too busy or off-line, then it will keep on trying.

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But if the computer does not exist then it sends an error message telling you that it can not reach the address you supplied and gives up. It is up to you to check what you typed and resend the message.

'Thinking' message

Unfortunately many e-mail systems try to be a bit cleverer than this. Although none that I know of try to fix spelling errors, lots of them will do something that can be far worse.

I have received architectural plans, personal letters about trips to Canada and once an image of someone's favourite dog

If you send a message to someone without giving them a full e-mail address then the mail system will assume you want to send it to someone who connects to the net through the same ISP.

So if you use the Pipex Dial service and send a message to Bill it will get to me because I have the address [email protected].

This is fine and can be useful, until you try to send a message to your friend [email protected] and inadvertently type bill,[email protected].

When you do this your mail program will send two messages, treating the comma as a separator between them. One will go to [email protected] and probably get bounced back. And the other will come to me.

Barbecue invitation

In my many long years online, I have received architectural plans, proposals for changes to the circuitry of the Sky Digital set-top box, personal letters about trips to Canada and once a four megabyte image of someone's favourite dog.

Be careful before you press the send button

I've been invited to barbecues in France, been told to get my comments on a database design in urgently and been privy to all the gory details about what happened at someone's birthday party.

When I get one of these e-mails I reply to it and tell the people concerned what has happened. It seems polite to do so, especially where someone may be waiting for a reply from the intended recipient.

This sort of mistake is very common because lots of people are sending e-mails and many of us are bad at typing.

Palace oops

It goes some way to explaining why I feel some sympathy for the as-yet-unnamed official in the Foreign Office who accidentally sent 55 pages of secret and potentially very compromising details of the plans for the Prince of Wales' recent visit to Poland to the wrong person.

Prince travel details revealed in an e-mail

Instead of going to an official at St James's Palace, the Prince's official residence, they went to a businessman who, after he had got over his surprise, told the newspapers.

The message probably came with a disclaimer like the one the BBC puts at the end of every outgoing e-mail. It says if you have received [this e-mail] in error, please delete it from your system, do not use, copy or disclose the information in any way nor act in reliance on it and notify the sender immediately.

These requests have no legal weight, of course, although e-mails are copyright and so you cannot just publish anything you receive.

But there is nothing to stop you acting on them. If I'd felt like flying to Nice for the barbecue I would probably not have been welcome, but I would not have broken any law.

Encrypting e-mails

A big part of the problem is that e-mails are like postcards - anyone can read them.

Encryption, the process of taking a message and turning it into a cipher, or secret, is simple, easy to use and widely available

This includes the people running the mail systems, of course, but it also includes anyone who inadvertently gets sent a copy, and every net user needs to realise this.

So while I can sympathise with the Foreign Office for making a simple error in sending an e-mail to the wrong address, I cannot understand why the travel details, even if not the whole e-mail, were not protected from prying eyes in some way.

Encryption, the process of taking a message and turning it into a cipher, or secret, is simple, easy to use and widely available. It is so common that the British Government passed a law - the Regulation of Investigatory Powers Act- to control its use and give police the power to force people to decrypt, or uncover, secret messages they send or receive.

Programs to encrypt and decrypt e-mails and files are simple: one of the best, Pretty Good Privacy, works seamlessly with e-mail programs so you don't even need to do anything complicated.

All you need to do is decide to use it, create two special keys that the program will use, and tell your friends you've started encrypting your private messages.

If the government is serious about using the internet, and if they want to replace diplomatic messengers with e-mail, they need to use it properly.

Sending a sensitive, unencrypted e-mail to anyone outside the government's own secure network is the electronic equivalent of having the Foreign Secretary take a memo down to Whitehall and ask a random passer-by to drop it in at St James's Palace on their way past. It is simply not good enough.

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