Enabling External Users to Book Exchange Room Calendars

When you live in a shared office space and share your meeting rooms with other companies, you might find yourself in a situation where your bookings of the shared meeting rooms can be a nightmare.

Especially if don't consolidate the bookings and meetings in one meeting room calendar and have access to that room from your Outlook Calendar.

But how do you do that? And how do you ensure that the meeting room can be booked from different Exchange environments, so you don't end up with double bookings, interruptions and meeting anarchy?

Two of more solutions could be:

  1. Let one company's Exchange environment be "master" and add the meeting rooms to this Exchange Server.
  2. Add a new Exchange environment (An Exchange Online tenant will do the job) and add the meetings room calendars to this new shared Exchange Online tenant (still following the rules about how meeting rooms should be configured - Learn more about that here.

For both, the solution is to allow "External Bookings" in the meeting room calendars. This will allow external senders to book a meeting in an Exchange Meeting Room Mailbox, which we are looking for here. 

In the case where we use the one company's Exchange Server as "master," this will allow the other companies in the shared office space to book the resources.

In the other case, the solution is quite the same, but now the meeting room resources are totally separated.

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 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.