I’ve recently had to migrate a client from Exchange 2010 to 2016 and quickly noticed that Outlook Anywhere no longer worked after redirecting Outlook Anywhere and other services such as autodiscover and webmail to the new server. Outlook Anywhere continued to work for users migrated to Exchange 2016 but not for users still on the legacy Exchange server. Using the Remote Connectivity Analyzer (https://testconnectivity.microsoft.com/) Outlook Connectivity feature would fail and throw the following error:
Attempting to send an Autodiscover POST request to potential Autodiscover URLs.
Autodiscover settings weren't obtained when the Autodiscover POST request was sent.
I was unable to find an official Microsoft KB which described this issue but I was able to come across this blog post for migrating from Exchange 2007 to Exchange 2013:
Exchange 2013 to 2007 Outlook Anywhere Proxy Issue
… and I can confirm that after adding the registry keys onto the Exchange 2010 servers as such:
… then restarting the servers was able to correct the Outlook Anywhere problem for Exchange 2010 users during the Exchange 2016 migration: