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Background : Free / Busy not working from O365 to On-Premise Exchange

While helping a client test their new Office 365 hybrid config today, we ran into an issue with free busy lookups not working from O365 to on-prem Exchange.
I ran through all the usual troubleshooting steps including the remote connectivity analyzer and even followed this article:
https://support.microsoft.com/en-us/help/10092/troubleshooting-free-busy-issues-in-exchange-hybrid-environment
When nothing out of the ordinary turned up, I logged a ticket with MS Support.
After 2 hours on the phone with Microsoft, they too couldn’t identify the issue and finally asked me to send them rafts of output for them to debug offline.
Now I’ll admit that I get a bit obsessive when I can’t resolve a technical issue, so when I got home I couldn’t leave it overnight so I decided to take another stab at resolving this issue. That’s when I stumbled across the issue.
O365 OWA didn’t display any error message when trying to view an on-premise mailbox calendar, it just shows the unavailable (greyed out) calendar.

 
but Outlook does !

Cause : Outlook displays error message “The external recipient’s server could not be determined”

Even though we’d spent half a day troubleshooting this and nothing had been uncovered I couldn’t leave it rest !

After seeing the message in Outlook again……a dim and distant memory resurfaced – of a time when after running the Hybrid configuration wizard, IIS doesn’t commit all the changes, and even though the RCA tool reports everything is ok, it’s not !

Resolution : IISRESET CAS Servers

So I jumped onto my CAS servers and just for the hell of it…. I performed an
IISRESET
in a command prompt window……wouldn’t you know it, the darned thing started working straight away !
Sometimes I hate computer…I really do !
🙂
 
*Word of caution, IISRESET can cause temporary outage on CAS servers so I wouldn’t do this during the middle of the day !