The email in question was from the lawyer for the House Ways and Means committee and contained an email that divulged the court clerk's
personal email, apparently not her work email. Federal employees are prohibited from using their personal email for government business (Hillary? Anyone?) except under very narrow exceptions, and they must copy or forward the email to their government account under the Federal records act. For instance, two Air Force employees cannot discuss offers from Lockheed and Raytheon on their private email, since in the event of a challenge of an award, all government records concerning the award are subject to discovery.
What Hillary did, running her Secretary of State's office out of a private email account was a playground for corruption and abuse, which is why those emails will never see the light of day. What is even worse, or weirder is that Huma Abedin used her sicko husband Anthony Weiner's laptop and left the entire mess there for the FBI to find. But never divulge.