Legal experts pushed back on the claim that Mueller illegally obtained the emails in question, some of which belong to senior adviser Jared Kushner.
"Of course Mueller obtained emails from a third party," wrote former federal prosecutor Renato Mariotti. "Prosecutors in most white collar criminal investigations do that. It's not 'inappropriate' or even unusual. Anyone who claims otherwise has no idea what they're talking about."
He added that it seemed like Trump's lawyers were just "playing politics," but that the development was nevertheless a "bad sign" for them.
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.