If you’re having issues with Free / Busy information with your on-prem users where they can’t see online user’s calendar information and have been through Microsoft’s Free / Busy troubleshooter (among countless other blog posts) without success, and are seeing this error:
It could be because TLS1.2 isn’t enabled on your Exchange servers.
You can run the script that is located here to enable TLS1.2 on your servers and remove older ciphers:
https://www.hass.de/content/setup-microsoft-windows-or-iis-ssl-perfect-forward-secrecy-and-tls-12
You may have to go into the registry for TLS1.2 and enable it for both Client and Server after running the script on your Exchange servers and rebooting them, then reboot again, but it resolved my issues after searching for weeks for the issue. I couldn’t find anything that specifically had that error: “The external recipient’s server could not be determined. Contact your administrator.”
Enabling TLS1.2 seemed to fix this issue immediately. However, some users needed to delete their autocomplete entry from Outlook before it worked properly.
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