By default an Exchange Server room mailbox does not permit external senders to make bookings. However there is an option that you can configure to allow external senders to make bookings if you need them to.
In this example scenario there are three room mailboxes.
[PS] C:\>Get-Mailbox -RecipientTypeDetails RoomMailbox Name Alias ServerName ProhibitSendQuota ---- ----- ---------- ----------------- HO Meeting Room 1 homeetingroom1 ex2013srv2 Unlimited Sunset Room sunsetroom ex2013srv1 Unlimited HO Meeting Room 2 homeetingroom2 ex2013srv2 Unlimited
Looking at the “Sunset Room” calendar processing settings we can see that the mailbox is not configured to process external meeting requests.
[PS] C:\>Get-Mailbox "Sunset Room" | Get-CalendarProcessing | Select *external* ProcessExternalMeetingMessages : False
If an external sender such as a Gmail user 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:
[PS] C:\>Get-Mailbox "Sunset Room" | Set-CalendarProcessing -ProcessExternalMeetingMessages $true
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 availability of the room and any other booking policies you have configured. 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.