The Non NDA NDA
blog.weatherby.net
blog.weatherby.net
How could they be legally binding?
As someone else has pointed out, you'd think the mere fact that someone auto-appends these things to every email diminishes the likelihood that a court would take them seriously.
Consider also the absurdity of the sender using an inherently insecure communications channel to communicate information with a demand that the receiver keep it confidential...
They're also overly broad, often claiming copyright over quotes or even content I just sent them!