E-mails may be knocked out in a hurry, but as evidence to the Hutton Inquiry proves, their contents can come back to haunt.
By Mark Ward
BBC News Online technology correspondent
E-mail is like an elephant. It never forgets.
No-one can remember what they said in every e-mail they ever sent, nor all the subjects they joked about or the people or events they passed comment on.
But a bad memory, either feigned or real, is no defence in these days of electronic archiving and back-up tapes.
As Alastair Campbell, Andrew Gilligan, Bill Gates, Kenneth Lay and many others have found out, long sent and presumed lost e-mail messages can come back to haunt you.
Only last week a woman received £10,000 in compensation from financial adviser Holden Meehan after discovering colleagues were exchanging obscene messages about her.
Even if you regularly purge old e-mails from your inbox and clean out the "deleted items" folder, it makes little difference.
The e-mail bin is not as final as the wastepaper basket
It is very likely that there are several copies of any message that you have sent, even if it is just to a colleague in the same office, sitting out there in cyberspace.
"If you are going to find a smoking gun, you will find it in e-mail," says Gordon Stevenson, managing director of computer forensics firm Vogon.
"There's always something," he says.
Part of the reason that e-mail is proving so useful in so many legal cases is because of the informality it encourages. Like a conversation around a table in the pub it encourages people to drop their guard.
"It combines the convenience and the invitation to be open that a telephone conversation does with the permanence of the written word and that's quite a trap," says Simon Chalton, an expert on data protection and computer law, and a consultant to law firm Bird & Bird.
And there is a lot of e-mail around.
Research by the Radicati Group, which focuses solely on messaging, found the average user sends and receives seven megabytes, about 110 messages, every day.
Evidence to the Hutton Inquiry
Part of this figure is down to spam, or unwanted commercial e-mail, some of which tends to be heavy on the graphics and web formatting.
The rest is generated because e-mail messages are easy to write and send.
Also growing use of the net has meant they have become an essential business tool as well as a boon to many people's private lives.
But this ease of use comes with a price. It means that exactly what you said is preserved and events can be seen in their chronological order.
When a message is deleted, it does not immediately disappear. Rather, the computer writes a tag that instructs the message to be overwritten if that chunk of memory is needed.
The sheer capacity of modern hard disks means this is, indeed, a rare event. The same goes for company servers, which will retain a copy of sent and received e-mails for months.
This can short-circuit court cases that revolve around disputes over a sequence of events, says Mr Stevenson.
Even this won't erase all traces
"If you go back and lift the back-up tapes you will be able to see if there was correspondence of action," he says.
Many firms are preserving e-mail messages for longer and longer.
Andrew Barnes, a spokesman for archiving firm KVS, says regulations in many business sectors demand that e-mails be kept along with other documents.
British companies are required by law to keep all business records, including working papers, for six years. As many working practices and contracts are prepared via e-mail, companies should really be archiving these too.
In the US similar regulations govern many different trades. For instance rule 3010 of the National Association of Securities Dealers code of conduct says that firms must supervise staff and ensure they are not trading stocks and shares unlawfully.
In December 2002, five investment banks were fined $8.25m for failing to comply with this rule.
Many firms now filter e-mail to stop viruses and spam and trawl through the rest for things that should not be there.
Incompetence best defence
In the process, records of those e-mails are logged and can form part of the jigsaw that contributes to evidence in a court case.
The good news for anyone that thinks they may have something to hide is that most firms are not being very diligent.
A survey carried out KVS found only 19% of the firms questioned had any reliable way of recovering e-mail more than a year old.
Enron's Kenneth Lay, confronted with e-mail evidence
Work by Osterman Research found that the average age of the oldest e-mail that can be retrieved is 10 months.
However, says Mr Stevenson, even then there are innumerable ways to recover data that was supposedly deleted. The fact that mail tends to pass through several different servers before it reaches its destination means that there could be several different copies of it in existence.
Again, segments of the e-mail, such as "subject" lines, times and dates of sending are retained and can be pieced together.
Using webmail can make a difference, especially if few people know you have the account. But even with webmail, the host can be forced to surrender records for a court case which may contain details of what you have written.