Many are still under the illusion that deleted e-mails vanish into the ether

For any competent systems operator, finding the e-mail that was sent through the Department of Transport's mail servers would have required a couple of hours. Most of that would have been spent narrowing down the time at which the e-mail was sent.

In this case, as the time was more than a week ago, the operator would have to have found a back-up magnetic tape (such as those used in recording studios) from the day of sending, and type a few obscure commands – for example "grep Pam Warren | echo $term" – on the keyboard. Hey presto, the computer screen would have displayed the incriminating evidence, a list of every e-mail with the words "Pam Warren".

While many senders think e-mails are evanescent, to an IT department they are data, and potentially valuable. They are stored, in the same way as valuable data such as accounts.

When an e-mail leaves your machine, it passes through a mail server, where, typically, a copy is made on to a back-up disk in another location, just in case there are problems.

The message would then pass through an internal network or the internet to reach the destination mail server, where the same duplication happens again, in case of further problems, and the recipient is informed that a message has arrived. Even if the sender and recipient delete the e-mail at once, at least two copies will remain on those back-up disks. Those are usually retained for up to three days but, typically, nightly back-ups are made on to a tape or recordable DVD, in case of catastrophe.

Stored e-mails have trapped the famous and infamous; perhaps the earliest case was that of Oliver North in the Iran-Contra scandal in the Reagan years. Bill Gates of Microsoft, Enron and the accountants Andersen have been among the many other victims.

Government rules require copies of e-mails to be kept in a semi-permanent electronic form, so they can be passed to the Public Record Office 30 years later. There was no chance that the Pam Warren e-mail, once sent, could be wiped from history.