Recently, we came across a strange issue in our Exchange hybrid environment. A few users with Outlook 2016 (Classic) were unable to view the free/busy information of a meeting room. At first glance, everything else seemed to be working fine — mail flow was good, room booking worked, and free/busy was visible in OWA and newer Outlook versions.
But when someone using Outlook 2016 tried to schedule a meeting with that room, all they saw in the scheduling assistant was a sea of grey. No calendar data. No availability. Nothing.
What We Checked First
Like with any Exchange issue, we started with the basics:
- Permissions on the room mailbox? All good.
- Organization Relationship settings? FreeBusyAccess was enabled.
- Federation Trust? Tested using Test-FederationTrust — passed every check.
So far, everything looked perfect on paper.
Digging Deeper
That’s when we thought — what if this isn’t a config issue, but a protocol or authentication problem?
We checked the EWS virtual directory on the DR Exchange server where the mailbox database was mounted. And guess what?
Basic Authentication was disabled
This was a lightbulb moment.
In many environments, especially after running the Hybrid Configuration Wizard, Microsoft Exchange tends to disable BasicAuth by default in favor of more secure methods like Modern Authentication (OAuth). But here’s the catch:
Outlook 2016 (Classic) doesn’t always play nicely with Modern Auth — especially if it’s not fully enabled across the tenant or clients.
The Fix
To test this theory, we re-enabled Basic Authentication on the EWS virtual directory where the mailbox was hosted:
Set-WebServicesVirtualDirectory -Identity "DR-WEX-MAIL01\EWS (Default Web Site)" -BasicAuthentication $true
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We did this only on the required server — not across the board — and immediately saw results.
The Result?
Outlook 2016 clients started showing free/busy status for the meeting room again.
Problem solved.
Final Thoughts
This was one of those classic hybrid Exchange quirks. Everything looks right, but legacy clients can still throw curveballs. If you’re running a mixed environment with older Outlook clients, don’t forget to check authentication settings on your virtual directories — especially after running the hybrid wizard.
Also, while BasicAuth worked for us here, it’s worth planning to move away from it entirely as Microsoft continues to phase it out. Enabling Modern Authentication across your organization should be the long-term goal.