I just heard a story about this last week. 8000 personal gmails accounts were voluntarily given to the feds upon request. Technically the ISP can require a warrant before releasing personal emails of their clients, but often they will just release them if they don't want a struggle with the government.
They made a great analogy during the story. They said your email account is like your high school locker. You may store your stuff in that space, but the space belongs to someone else and they can access it whenever they want.
I'm not quoting idiots who promote unsafe recovery strap techniques anymore. :miff: