It's official [in USA]: emails are like postcards

What many of us forget on a day to day level, is that emails simply pass through a whole lot of servers before they get read by their intended recipient. It is not impossible to keep copies of what passes through a server and to read it later. Thus, I was always told, treat an email like a postcard; don't put anything so private in it that it won't stand being read by a third party.

The court ruling in Massachussets enshrines it as a legal principle.Though the 'ps' is that it may infringe a stored communications act in USA. Don't know what the UK position is.
So, just a reminder: be careful: no credit card numbers in your emails.