The disclosure came in a letter sent to two congressional committees by Kory Langhofer, general counsel of Trump's still-existing transition group, Trump for America.
In the letter to the Republican heads of the House Oversight and Senate Homeland Security panels, Langhofer said Mueller's investigators obtained the emails from the General Services Administration, a federal agency that stored the material, rather than requesting them from the transition organisation.
While conservatives have been critical of Mueller's probe of Russian activities during the 2016 campaign, Trump said Sunday afternoon that he has no plans to fire Mueller.
The president did criticize the fact that Mueller had gained access the emails, however. Trump said it was "not looking good" and again stressed that there was "no collusion" with Russia an important question the probe is examining.
More From This Section
The tens of thousands of emails in question pertain to 13 senior Trump transition officials. Many of the emails that Mueller's investigators have now include national security discussions about possible Trump international aims as well as candid assessments of candidates for top government posts, said those familiar with the transition.
They spoke to The Associated Press on condition of anonymity because of the records' sensitivity.
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.
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.