You might find that a Resource mailbox with no delegates does not accept a meeting request but tentatively accepts it.
To check and fix this we need to do the following:
Open up the Exchange Management Shell and run the command below:
- Get-CalendarProcessing “Name” | fl
- Check if the “AllRequestOutOfPolicy” is set to $True, if it is then we need to run the command below to set it to $False.
- Set-CalendarProcessing “Name” -AllRequestOutOfPolicy $False
- After this the meeting requests should be accepted.
Hope it helps.