"Hillary Rodham Clinton exclusively used a personal email account to conduct government business as secretary of state ... and may have violated federal requirements that officials' correspondence be retained as part of the agency's record." - Michael S. Schmidt, The New York Times, March 2, 2015. Here is the article.
The fact that Hillary Clinton has become the target of just about everything which could be investigated about her past demonstrates what a formidable politico she has mutated into. The Stop Hillary movement just won't stop.
That's why it would be difficult to discern from media reports if her use of personal email endangered the security of government communications and how much so. For this, the trustworthy expert to go to might just be Ed Snowden. The documentary "Citizen Four" positions him as an objective observer of security measures. He can separate excessive scrutiny from what's necessary for national safety.