An exchange user tries to create a meeting request and selects a resource mailbox as another attendee or a resource. The next thing they know is that availability is grayed out. What do you do?

I get this question quiet a bit especially from users who our team just created a resource (room) mailbox.

Here is the scoop. Free/busy publishing process relies on both Outlook (MAPI) client and OWA server to interact with Exchange server public folder store in order to publish any initial appointments and further any changes to the calendar. Thus, for users who just got a brand new mailbox, my answer is simple. Schedule an appointment and see.

Markus Klein from msexchange.org delves into the details of free/busy generation process, here.

However, sometimes your free/busy data does not show. The first step is to run Outlook with "/cleanfreebusy" switch. It forces Outlook client to publish its mailbox calendar data onto a server. From technet's article, I can locate the actual message in Public Folder hierarchy. After picking Eric Hartmann's brain for a bit at here, it becomes very easy to audit the process. All of the sudden, free/busy data is no longer magic.