You may experience a problem where you are unable to book resource mailboxes across forests whose Global Address Lists have been synchronized by Collaboration Services (QCS). The meeting requests are not auto-accepted.
However, meeting requests from users in the same forest as the resource are auto-accepted successfully.
By default, Microsoft Exchange will not auto-accept meeting requests from outside the forest.
To resolve this issue, install the Microsoft Exchange Auto Accept Agent on the Exchange servers in each forest:
This agent will auto-accept the meeting requests and resolve the problem.