Upgrading from Exchange 2010 to Exchange 2016. Everything is connected and communicating. However, whenever we point the external interface to the Exchange 2016 server, Outlook anywhere for any mailbox on the 2010 server fails to connect. OWA and Activesync works just fine. Internal connectivity is fine. Just Outlook Anywhere. I have gone through the co-exist configuration, setting Outlook Anywhere to an external mail.crestoperations.com, which is my external DNS to our NAT firewall (the only thing I change is the NAT pointer). I have configured Outlook Anywhere on the 2016 server to use NTLM. I have configured IIS on the 2010 server to use both basic and NTLM. Any advise on configuration would be appreciated.