UPDATE: A lawyer for Trump for America (TFA), President Trump's transition team, is pushing back on Mueller's acquisition of the emails. However, a spokesman for the special counsel's office confirms Mueller obtained them legally:
On Saturday, Kory Langhofer, general counsel for the transition group, sent a letter to two congressional committees arguing that the GSA had improperly provided the transition records to Mueller’s investigators. In the letter to the Republican chairmen of the House Oversight and the Senate Homeland Security committees, Langhofer contends that the disclosure by GSA was “unauthorized,” and it considers the documents private and privileged and not government property.
Langhofer also said that a GSA official appointed by Trump in May had assured the transition in June that any request for records from Mueller’s office would be referred to the transition’s attorneys. According to Langhofer, the assurance was made by then-GSA General Counsel Richard Beckler, who was hospitalized in August and has since died...
But late Saturday, another GSA official present for the conversation told Buzzfeed News that there was nothing improper about the disclosure of the emails to Mueller’s team. The GSA has provided office space and other aid to presidential transitions in recent years and typically houses electronic transition records in its computer system.
GSA Deputy Counsel Lenny Loewentritt told Buzzfeed that Beckler didn’t make a commitment to the transition team that requests from law enforcement for materials would be routed through transition lawyers.
Specifically, Loewentritt said, "in using our devices," transition team members were informed that materials "would not be held back in any law enforcement" actions.
Loewentritt read to BuzzFeed News a series of agreements that anyone had to agree to when using GSA materials during the transition, including that there could be monitoring and auditing of devices and that, "Therefore, no expectation of privacy can be assumed."
Loewentritt told BuzzFeed News that the GSA initially "suggested a warrant or subpoena" for the materials, but that the Special Counsel's Office determined the letter route was sufficient.
“When we have obtained emails in the course of our ongoing criminal investigation, we have secured either the account owner’s consent or appropriate criminal process,” Peter Carr, a spokesman for the special counsel’s office, said in a statement to The Hill.
Mueller team defends obtaining Trump transition emails (The Hill)
Special counsel obtains thousands of Trump transition emails (AP)
Key Officials Push Back Against Trump Campaign’s Claim That A Federal Office Illegally Turned Over Emails To Special Counsel (Buzzfeed News)
According to Axios, Special Counsel Robert Mueller has obtained tens of thousands of Trump transition team emails from 13 different accounts by going through the General Services Administration, the government agency that hosted the transition email system.
Why it matters: The transition emails are said to include sensitive exchanges on matters that include potential appointments, gossip about the views of particular senators involved in the confirmation process, speculation about vulnerabilities of Trump nominees, strategizing about press statements, and policy planning on everything from war to taxes.
Axios also reports that transition officials, in anticipation of Mueller's inquiry, "separated the ones they considered privileged," but because the special counsel has the full cache from the GSA, he will see everything.
Scoop: Mueller obtains "tens of thousands" of Trump transition emails (Axios)