Yes, that could be. I don't know either how the day-to-day activities of the State Department work. I would think that the Secretary of State may have need to communicate with Station Chiefs throughout the world and others in Washington while she traveled. In this day and age so much business communication is performed via e-mail. According to Hillary she sent an average of 20-30 business-related e-mails on her personal server per day throughout her tenure as Secretary of State, but none of those discussed classified information.
It could be that all classified info was communicated to her verbally or by hard copy. Though it seems odd that not once in four years did anyone want to send a file attachment to her.
As a (very) loose basis of comparison, my brother worked for the FBI in a Foreign Counter Intelligence group pre- and post- 9/11. He was not allowed to have anything on his personal e-mail, could not use his bureau car for any personal business, and had to use an encrypted cell phone for business. It may be very different from the State Department, but the FBI drilled into their employees the importance of the fact that their job dealt with sensitive information.
That's a very good question. Anyone sending an e-mail to her should have recognized the e-mail address was different from others in the State Department. I'm assuming that her personal address was something like <
[email protected]> whereas if she had a business e-mail it would have been <
[email protected]> or something like that.