1) The IP channels to this email host were not secure.
2) The server itself was not secure.
Just item 1 is enough to compromise any and all transmissions, since the right spyware at the ISPs hub could sniff and copy every packet sent in real-time to a separate location for storage. The NSA uses this already, with the ISPs complicity; foreign actors would have to sneak this in, and do.
One main lesson from this is that Hillary put her personal privacy needs far above the nation’s national security.
And she could still be elected president. God help this nation of morons.