is a SIPRNet terminal always
inside a SCIF?
I want to know who,
among Hillary’s aides, stripped the headers off of the classified stuff.
No, it's not necessary. A SCIF is for Sensitive Compartmented Information, which not on the SIPRNet.
The DoD NIPRNet (.mil) is equivalent to State's OpenNet (.state.gov). The DoD SIPRNet (.smil.mil) is equivalent to State's ClassNet (.state.sgov.gov). The SIPRNet and ClassNet can reach each other.
State has no equivalent to JWICS, so they use that for TS/SCI.