Great article from WSJ covering this yesterday:
https://www.msn.com/en-us/news/politics/mueller-probe-faces-new-challenge/ar-BBGVTRL
In the Saturday letter, Mr. Langhofer said that a lawyer with the General Services Administration, a federal agency that helped set up the transition office, acknowledged in June that the Trump transition organization owned and controlled the emails, and that “GSA had no right to access or control the records.”
Mr. Langhofer’s letter cited a Justice Department document from 1988 that found that presidential transition teams “are not federal agencies” subject to the federal Freedom of Information Act as well as a 1983 case that found transition documents were “personal records” rather than “agency records.”
He did point out that with these emails Mueller's team will be able to compare what Trump transition team members said in emails to what they said when interviewed by the FBI (some ended up with jobs in the White House) so they could find them guilty of lying to the FBI, like in the case of General Flynn. Of course that could be faulty memories but the FBI can call it lying...the old "perjury trap."
Final paragraph:
If Mueller wants more than a coup stick prosecution, the move against the GSA was one door he should have opened rather than kicked down in his investigation. Only time will tell, of course, but this is a legal complication that was as unnecessary as it was unprecedented.
The transition lawyers insist that Richard Beckler, general counsel for the GSA, reportedly agreed with transition officials that this information belonged to the transition team and that GSA had no right to access or control the records, but GSA denies the comment. The Trump lawyers argue that, when Beckler was hospitalized, Mueller’s people moved on the seizure and acquired the thousands of messages.