Bookings Mail Flow and eDiscovery

We are running a POC with MS Bookings. We have sent various invitations from test calendars but none of these mails are showing up in eDiscovery searches. In fact, the scheduling mailboxes are not available for selection in an eDiscovery search. We are wondering how ails are sent form MS Booking when a customer is using Exchange Hybrid. Are the mails not sent form the scheduling mailbox that backs the booking calendar? If so, why are these mails not in eDiscovery?