The concerns for Hillary Clinton's private email server were perhaps well-founded. According to Fox News and Politico, two of the emails on that server were labeled as "top secret."
According to those sources, whether or not the two emails were classified at the highest level is a "settled matter."
One of the emails was eventually reduced to the level of "secret," however, obviously, critics suggest that wasn't really the point. The point is that these emails weren't subject to the tough government security standards, so they could have been compromised.
Of course, whether or not the emails end up being a legal issue for Clinton and her presidential campaign is another question entirely. The situation is still under investigation by the FBI, which is working to determine whether or not Clinton violated the law by running her own email server instead of using official servers. If the FBI does find what Clinton did to be illegal, it could completely derail her campaign, but if the finding is that she didn't really do anything wrong, then it could be a boost for her.