Illegal - nope. Not illegal. Private email however was a gray area virtually every one of her predecessors used. Not specifically prohibited but not exactly compliant either.
Not going to replay all this during market hours. but bullshit/all to all this replaying history again to have another bite at the apple in getting people to buy the hillary apologist view.
The problem was not private email. The problem was that she was doing state department related business on the private email which was not being backed up. Then she destroyed what was directed requested by Congress.
And yes, early days, Colin Powell did this and Colin Powell did that in the early days. But it was settled by then and she was required to back up all copies in a manner that was accessible to the state department. The other rather obvious thing is that the citations of others who violated that requirement in the early days refer to some emails here and some emails there. But she was running an entire offline state department communications system from her own server for crisesake.