From the parent of my original comment:
Almost every SMTP server out there today requires authentication ... That is the very definition of trying to keep things private.
Do you disagree with me? Do you believe that SMTP authentication contributes to privacy and not authenticity?
If I am using them to communicate with a 3rd party, I have a reasonable expectation of privacy between myself and that 3rd party. You would most certainly need a warrant to turn around and try to get access to a message stored on their servers.
On top of that, quite a bit of email today doesn't even touch SMTP. If I'm sending an email from one GMail user to another GMail user, I'm pretty sure it is just shuffled around on Google's internal servers. And, of course, I'm connecting to Google using SSL, an encrypted connection.
How does that not scream private?
I cut out the SSL and TLS because I don't disagree; SSL and TLS certainly says "privacy". I wanted to specifically address the notion that "authentication =~ privacy"
Why can't it be both?
Why should storing something on a third party's system obviate the need for a warrant? If I rent real property and use it to store my papers, does that mean the government should be entitled to seize them without a warrant because I'm storing them on the premises of a third party? If not, what's the difference?
Email being transmitted by 3rd parties is not different from voice calls being transmitted by 3rd parties. Yes, you are trusting a provider, with the expectation that your provider will send the data where you've asked it to send the data, and nowhere else. This is still true whether you're talking postal service, landline voice calls, SMS, cellular voice calls, Skype, etc.