"I thought it was allowed. I knew past secretaries of state used personal email," she told CNN's Wolf Blitzer on "The Situation Room, saying the rules were only recently improved.
"They were not a model of clarity and it seems like there is still more work to do on that," she said.
Clinton reaffirmed that she had made an error in using a private email server, saying, "It was still a mistake. If I could go back, I'd do it differently."