Did she use SIPRNET or other secure system for comms? Of course nothing was marked classified if she was using her own non-secure server. That would be a huge red flag, that no one could ignore, this should secure her position in jail.
Did she use SIPRNET? Of course not!
Her email server used plain ole SMTP. Relays all over the world, easy-peasy gathering of info for any intelligence service worth a wooden nickel to use.
I would consider everything that passed through “clintonemail.com” to be thoroughly compromised. There’s really no other way to look at it.