Ping
From Day One, Clinton was never assigned and never used a state.gov e-mail address like previous secretaries.
“That’s a change in the standard. It tells me that this was premeditated. And this eliminates claims by the State Department that they were unaware of her private e-mail server until later,” Krongard said... “How else was she supposed to do business without e-mail?”
IOW, this was planned from day one as a means of controlling access to communications relating to official (or 'unofficial') business.
That adds premeditation and conspiracy to the other charges--she didn't make that choice nor implement it alone.