Good question, Huma

Politico: "Obama used a pseudonym in emails with Clinton, FBI documents reveal"

"Once informed that the sender's name is believed to be pseudonym used by the president, Abedin exclaimed: 'How is this not classified?'" the report says. "Abedin then expressed her amazement at the president's use of a pseudonym and asked if she could have a copy of the email."

Well, it wasn't marked classified so of course Hillary wouldn't know if communication from the President of the United States to the Secretary of State would be classified.  Who could determine such a thing without a big red C on the page, which is probably just a paragraph identifier anyway.