How to Enable External Users to Book Exchange Room Calendars
By default, an Exchange Server room mailbox does not permit external senders to make bookings in the calendar. However, there is an option that you can configure to allow external senders (people from outside your organization or people with email addresses on other domains) to make bookings if you need them to. For instance, if you live in a shared office space and share your meeting rooms, and you need to book the shared resources.
What you need to configure is the calendars "processing settings" using PowerShell. More specific you need to set the
ProcessExternalMeetingMessages to
true.
Looking at a meeting room resource calendar processing settings, we can see if the mailbox is not configured to process external meeting requests.
If it's
false and an external sender attempts to book the meeting room they will not receive an acceptance or rejection message, which may lead to confusion if the room is assumed to have been successfully booked.
To configure the room mailbox to process external meeting requests run the following command in PowerShell:
Set-CalendarProcessing -ProcessExternalMeetingMessages $true
Please note that this change will enable the
processing of external meeting requests, but the meeting request is still subject to being accepted/rejected based on the availability of the room and any other booking policies you have configured in your
scheduling permissions. This change will also only take effect for new meeting requests. Any meeting requests from external senders that were received before the setting was enabled will not be processed.