The thing that bothers so many long-time email users (me included) is the fact that email has never been a private communications method. This is nothing new. Unless you encrypt the content, it's no different from sending a postcard. Simply put, somehow over time, the public has been duped into thinking that email is private. Keeping with the Gmail theme of this forum, it kinda reminds me Gmail's "scanning" methods to provide content-relevent ads. It's a method that's been used by Spam managers and Virus scanners for many, many years, but finally the public gets a taste of just how it works, and they are up in arms. <stepping off of soapbox...>
I don't think you should worry about that too much, a huge service like gmail, yahoo or hotmail would not check peoples emails. But you should be cautious with smaller providers. Another precaution you should take is never enter confidential information on an email such as credit card numbers or social security numbers.