"Don't pass it on ...": have the French got it right or wrong?

"We'd better open them first,
just to make sure ..."

Reading one of his favourite IP weblogs, Mark Anderson's IP Draughts, this Kat was both intrigued and startled by one of the items on Mark's recent post "Ten things I have learned recently ...", here. Item no.2, handsomely and somewhat cryptically illustrated with a picture of half an avocado pear, replete with stone, reads thus:

"The approach of French lawyers in relation to confidentiality of communications between lawyers is very different to that of English lawyers. It seems that, in France, a lawyer may not copy to his client a letter that he has received from the other party’s lawyer. Result: effective communication with one’s clients becomes unnecessarily difficult".

The Kat is fascinated by this.  Knowing that this blog is read by a large number of thoughtful and highly capable French lawyers, he hopes that one or more of them will be tempted to respond to the following questions:

• Does this rule really exist?
  
• If so, is it a general rule of law that applies in respect of all professions (and therefore applicable also to, eg, accountants, architects, surveyors, loss adjusters), a rule imposed for the purpose of regulating the conduct of the legal profession or merely a matter of professional etiquette which has assumed the status of a binding rule?
  
• What if any are the sanctions that may be imposed upon a lawyer who copies a letter received by the other client's lawyer to his client? Are they civil or criminal, based in copyright or breach of confidence, data protection or privacy, or what?
  
• May any use be made, or cognisance taken, of such a letter if it has indeed been so copied?
  
• Does it apply in respect of all letters from the other party's lawyer or only those from a French-qualified practitioner?
  
• May this restriction be waived by the sender of the letter or by his client?
  
• Is any justification offered for this rule and is there any consideration as to whether it should be modified or scrapped?

Merpel's curious to know whether other jurisdictions impose a similar restriction upon the forwarding of professional correspondence. Do tell!

French letters here and here