They are gov't emails -- and they may not have even needed a subpoena, nonetheless:
Jeffrey Cramer, a longtime former federal prosecutor who specialized in white collar cases, echoed that point.
"This is not a problem," he said, referring to the way Mueller's team got a hold of the emails. "The server owner, in this case GSA, properly has the emails and can turn them over if there was a subpoena or court order," in the same way that internet providers and banks can provide emails and records about clients to law enforcement.
More than that, Cramer added, the special counsel's team may not even have needed a subpoena to obtain the emails. An administrative request — a legally authorized and judicially enforceable demand for records issued by a government authority — may have sufficed, he said.
It's unclear which officials the emails belong to and what they contain. But it's likely they will provide a number of new leads for the special counsel to follow. "
Mueller has obtained 'tens of thousands' of Trump transition team emails - Business Insider