The dispute has seen the inside of five courthouses since 1995, when Ms Smith first sued Marshall's son, e. Pierce Marshall, in a Texas state court, accusing him of cutting her off from the estate.

Lawyers and experts on Internet policy say no court case has ever turned on the presence or absence of such an automatic E-mail footer in America, the most litigious of rich countries.

Others say they don't put details into e-mails or instant messages because things might be taken out of context if they were ever to end up in court.