Hi folks,
Just wanted to share with you a quick way to check if your meeting rooms accept invitations from the outside of the Exchange organization.
$MBX = Get-Mailbox -RecipientTypeDetails RoomMailbox -ResultSize Unlimited
$MBX |select DisplayName,@{N="External Calendar Processing";E={(Get-calendarprocessing -identity $_.Name).ProcessExternalMeetingMessages}}
Enjoy.