Since the Clinton Administration the White House staff manual has said that no official government business should be done on personal email accounts. This point should be even more obvious with respect to classified information.
I remember saying that once or twice in White House ethics lectures. I wanted to stress even obvious points after the indictment and resignation of the Vice President’s Chief of Staff (Scooter Libby) in a scandal involving a leak of classified information. But no classified information in personal email? If the White House ethics lawyer has to tell people that, he might as well also warn people against stuffing classified information in their socks simply because President Clinton's former aide Sandy Berger did just that when illegally removing classified files from the National Archives. I realized that I was telling people something they already knew – which trivializes ethics lectures – and I dropped the point.
Then again, the State Department – a place where there is perhaps even more classified information than in the White House – apparently needs to bring this seemingly obvious point back into its ethics training: please don’t put classified information in personal email.
http://www.wsj.com/articles/investigation-sought-into-hillary-clintons-emails-1437714369
That – and perhaps also a reminder not to copy Vladimir Putin on any emails containing classified information . . .