I had someone who did SECRET-grade e-mails setup in the military write the following to me:
So, if for example Clinton only dealt with SECRET materials and they were sent or received in her email, all of the equipment (routers, switches, etc.) would have to be rated for that SIPRNet connection. Also, the space in which the equipment and servers and client computers resided in would also have to meet the specifications for SECRET material. This would include various forms of physical access to the space in the form of secure cards, biometrics, etc. No space rated for SECRET opens with a key from the local hardware store. . . .
The biggest issue I see here would be is if the server was connected to the public Internet and it resided in a non-DoD-approved space.
Not sure there are biometrics installed in the Clinton home in Chappaqua.