EDIT 9/19/2023: This blog post has received significant update.
In this second part of the Demystifying Hybrid Free/Busy, we will cover troubleshooting of Hybrid Free/Busy scenarios, more specifica...
Updated Sep 19, 2023
Version 16.0
Blog Post
Well hello, to everyone who likes old topics.... I'm wondering if anyone has any quick tips for a 2010/2016 co-existence customer with mailboxes residing on 2010, unable to view free/busy for migrated mailboxes.
- Cloud to on-premises works fine in all cases.
- On-premises (mailbox on 2016) to cloud works fine too.
- Just On-premises (mailbox on 2010) to cloud fails.
- OAuth is enabled
- Availability Address Space in place for {Tenant}.mail.onmicrosoft.com
- IOC in place as well.
- These things in place by HCW.
My suspicion is that it is related to TLS 1.0 / 1.1 being enabled on the 2010's (where TLS 1.2 is enabled too). There is hesitation in disabling TLS 1.0/1.1 on the 2010's without knowing it would solve this final Free/Busy issue. I'd like to find the evidence, just struggling to do so. Fiddler on the client side shows nothing, which leads me to believe it is 2010 server is trying something and failing. Also the issue is the same regardless of the client (Outlook/OWA).
My understanding based on the blog post here is that since OAuth is enabled, 2010 should use the Availability Address Space (for domain of the remote routing address of the remote mailbox being Free/Busy-lookup'd) to find the Exchange 2016 EWS URL which should lead to Exchange 2016 using the IOC to perform OAuth-based Free/Busy lookup, return result to 2010, which returns result back to user. Hopefully this is correct. Either way, looking for tips on this scenario and problem if anyone has any - thanks in advance.