I just got off the phone with a colleague who discovered his boss is reading his email. The employee, whom I'll call Dave, is hurt, unhappy, angry, and frustrated. “Yes, I know my email isn't private, but what did I do that would prompt my boss to read my email?” The more he talked, the angrier he became.
If you're a manager and you want to know what your employees are up to, I suggest reading email as a last resort. Of course, if you're concerned that someone is stealing, or using your company to do something illegal or unethical, reading email may be your only trail. But if you want to check on what an employee thinks or how their work is proceeding, talk to the employee.
In Dave's case, he's concerned about his (new) manager. This manager is not listening to his employees — changing estimates without regard to the realities of the situation, imposing his specific ideas on how people are supposed to work. Additionally, the manager wants to know who's in “his camp.” Managers who change estimates without regards to reality deserve what they get. I don't have much patience with managers who insist on telling people how to perform tasks when it's just preference. But why does a manager care who's in his camp? (What the heck does that mean anyway?)
If you want to know what people think of you, ask. If you really want to know. But don't read someone's email unless there's a darn good reason. Breaking people's trust (not about their email, about their ability to perform a great job) is not worth the aggravation. The more you trust people to do good jobs, the more they will.
Management isn't a beauty pageant or an eleection. If you're delivering results, and increasing capacity while continuing to work through and with people, you're probably doing a great job as a manager. You won't need camps or external approval — your job will speak for itself.