I just installed a single Exchange 2016 CU1 server in our test environment. I have two Exchange 2010 SP3 Update Rollup 12 servers in this site also. I can send an email from a 2016 mailbox to a 2010 mailbox but cannot send an email using the same client from 2010 to 2016.
The error in the Queue viewer on the Exchange 2010 server is "451 4.4.0 Primary target IP address responded with : "451 5.7.3 Cannot achieve Exchange server authentication.
Here is the strange part. I can logon to the Exchange 2010 servers and open a telnet session to the 2016 server and successfully send an email to the 2016 mailbox.
It's only when I send an email using the workstation, Windows 7 with Office 2016 and all updates installed, does the message get stuck in the Exchange 2010 queue. Very weird problem. I am very new to Exchange 2016 so I am sure I a missing something in the configuration.
I found some articles Googling but none seem to help.