I hear you, but that doesn't seem like how anything works in our legal system. When warrants are issued for cellphone records, text messages, financial records, etc, they don't have so narrow a scope.

The state has probable cause to believe there is evidence of criminal behavior in your <insert anything here>. Therefore the warrant gives them access to it in order to look for that (previously specified) criminal behavior. In this case everyone expects to find a smoking gun in his email - either talking to his manager or talking to someone else. That seems like probable cause to me.