Unless you run your own smtp and pop/imap mail servers, there is no way to be sure no one will read your emails. Even then, your servers could get hacked or maybe your contacts use a hosted solution (and most people do) that could get hacked, there is again no guaranty. The only work around is to fully encrypt your emails, even then someone could steal your private keys, or you could be using an unpatched encryption software etc.
It's a matter of trust and time saving. Do you trust google, yahoo employees etc to not spy on you more than you trust your ability and your contacts ability not to get your mail servers hacked? How much of your time and money are you ready to waste maintaining such a server?