https://www.gpo.gov/fdsys/pkg/USCODE-2011-title18/html/USCODE-2011-title18-partI-chap93-sec1924.htm
18 U.S.C.
United States Code, 2011 Edition
Title 18 - CRIMES AND CRIMINAL PROCEDURE
PART I - CRIMES
CHAPTER 93 - PUBLIC OFFICERS AND EMPLOYEES
Sec. 1924 - Unauthorized removal and retention of classified documents or material
From the U.S. Government Printing Office,
www.gpo.gov
§1924. Unauthorized removal and retention of classified documents or material
(a) Whoever, being an officer, employee, contractor, or consultant of the United States, and, by virtue of his office, employment, position, or contract, becomes possessed of documents or materials containing classified information of the United States, knowingly removes such documents or materials without authority and with the intent to retain such documents or materials at an unauthorized location shall be fined under this title or imprisoned for not more than one year, or both.
(b) For purposes of this section, the provision of documents and materials to the Congress shall not constitute an offense under subsection (a).
(c) In this section, the term “classified information of the United States” means information originated, owned, or possessed by the United States Government concerning the national defense or foreign relations of the United States that has been determined pursuant to law or Executive order to require protection against unauthorized disclosure in the interests of national security.
(Added Pub. L. 103–359, title VIII, §808(a), Oct. 14, 1994, 108 Stat. 3453; amended Pub. L. 107–273, div. B, title IV, §4002(d)(1)(C)(i), Nov. 2, 2002, 116 Stat. 1809.)
There are basically three different ways to transmit information within the State Dept.
Two of them are quarantined, secured and are not able to transmit outside themselves; Secret Internet Protocol Router Network (SIPRNet) and the Joint Worldwide Intelligence Communications System (JWICS) these are the systems used to send confidential, classified and top secret information. The fact that anything is on these systems means their content is classified.
The third is NIPRNet is able to send info to outside emails and such but is not connected to the other two. It is impossible to send anything from SIPRNet or JWICS to NIPRNet. It's set up that way on purpose so it cannot happen. The only way this can happen is for someone to willfully copy something from the first two and resend it which is forbidden. Everyone that has access to these systems knows this.
Yet over 1300 emails and memos that originated on either SIPRNet or JWICS ended up on Hillarys home commuter. The reason why most didn't have the original classification marking was due to the fact they were illegally copied and the classification markings left off.