Last I checked, the regs regarding emails for the state department went in to effect in 2005 and clarified in 2009 iirc.
There's also the Federal Records Act of 1950.
Under the Act, each federal agency is required to make and preserve records that (1) document the organization, functions, policies, decisions, procedures, and essential transactions of the agency and (2) provide the information necessary to protect the legal and financial rights of the government and of persons directly affected by the agency’s activities. The Act defines a federal record without respect to format.
The Act also requires agencies to maintain a records management program. Did Hillary have one? Based on what I have heard about the process used to determine which emails were "personal" and which were business it doesn't sound like she did.
Accordingly, to ensure that they have appropriate recordkeeping systems with which to manage and preserve their records, agencies are required to develop records management programs. These programs are intended, among other things, to provide for accurate and complete documentation of the policies and transactions of each federal agency, to control the quality and quantity of records they produce, and to provide for judicious preservation and disposal of federal records.