When we create a room mailbox, we can define if the room uses delegates or does an auto accept when possible. We can define a basic setting during the creating of the room mailbox. Do to define a resource mailbox, go to Admin > Exchange > Recipients > Resources and click on the +
When we edit the Room mailbox we have more settings.
But there is one little caveat. In this scenario every conflict will result in a Tentative instead of a Rejected meeting. The reason for this behavior is a little parameter called: AllRequestOutOfPolicy. By default this parameter is true. This effectively treats all conflicts as “Out Of Policy” and requires delegate approval for the meeting to be scheduled. This causes the “Tentative” email response.
You can only change this by using PowerShell.
Set-CalendarProcessing “Name of Room” -AllRequestOutOfPolicy $False
This sets it to false and automatically sends a declined email to the meeting requestor for all “Out of Policy” Meeting requests, which includes conflicts.