On that date did Podesta have a .gov address? If not then it was a private citizen to a Cabinet Sec about a future position. No if and I say if that was the case, the use of private email was more than appropriate it was correct.
Bob Gates contacted John Podesta to give him his private email address. I’m sure Gates had a government address. It looks like Gates initiated the contacts to his private email.
The email below is about official government business.
Gates had already sent a few emails to Podesta to use his private email.
From:r_m_gates@att.net
To: john.podesta@gmail.com