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.

“Mueller is using the emails to confirm things, and get new leads,” a transition source told me.

How it happened: The sources say Mueller obtained the emails from the General Services Administration, the government agency that hosted the transition email system, which had addresses ending in “ptt.gov,” for Presidential Transition Team.