“Care to join me in my courtside seats?”

Nearly 16 months ago, a batch of emails between the mayor and two supporters was released by City Hall, depicting the mayor’s chummy relationship with the men.

But the courtside-seat email, sent to the mayor from one of the men, Jona S. Rechnitz, was not among them.

That correspondence, showing a particularly transactional relationship between Mr. Rechnitz and the mayor, as well as another previously undisclosed email, emerged this week during a police corruption trial in Federal District Court in Manhattan.

Mayor Bill de Blasio and his staff are required to keep records that are connected to his government work. That includes emails about public activities that are sent to private email accounts. If someone requests the mayor to do something in his capacity as an elected official, those emails would be subject to public review.