I left out links. here's link to Cartwright questioning Comey (well, it will show the video, but if you right click on video a menu appears which has the option "copy video url". click on that and you will have the url which you can paste where you want to):
here is the link to a DAily KOS post which provides an excerpt of the key question from Cartwright and Comey's answer:
http://www.dailykos.com/story/2016/7/7/1546038/-Comey-tanks-key-GOP-talking-point-admits-classified-materials-were-not-properly-marked
MATT CARTWRIGHT: You were asked about markings on a few documents, I have the manual here, marking national classified security information. And I don't think you were given a full chance to talk about those three documents with the little c's on them. Were they properly documented? Were they properly marked according to the manual?
JAMES COMEY: No. [...]
MATT CARTWRIGHT: According to the manual, if you're going to classify something, there has to be a header on the document? Right?
JAMES COMEY: Correct.
MATT CARTWRIGHT: Was there a header on the three documents that we've discussed today that had the little c in the text someplace?
JAMES COMEY: No. There were three e-mails, the c was in the body, in the text, but there was no header on the email or in the text.
MATT CARTWRIGHT: So if Secretary Clinton really were an expert about what's classified and what's not classified and we're following the manual, the absence of a header would tell her immediately that those three documents were not classified. Am I correct in that?
JAMES COMEY: That would be a reasonable inference.
Also, I didn't include this in my comment in OP, but at the time Clinton was SoS, there were NO regulations of the State Dept. prohibiting use of personal email acccounts. repeat: NO rules or regs prohibiting use of personal email accounts for governemtn business. Use of personal email acccounts was 'discouraged', but not prohibited.
here is a good article on CLinton's email arrangements, how they came to be:
What the FBI Files Reveal About Hillary Clintons Email Server
14 Excerpts From the FBI's Report on Hillary Clinton's Email