I don't think it's ok by any stretch, but making it illegal might be a little pointless - how are you going to enforce it? And there's burden of proof and all that shit. It just seems an impossible thing to actually enforce.
What about after death? I've always thought of my email as the closest thing to an autobiography I've ever written, it seems plausible that people might be interested in reading your email after you pass away to gain insight into your life.
And I'm not implying anyone should be killed so their email is made public.