Exchange 2010 > 2016 mail flow

I am migrating a client from Exchange 2010 to 2016.  I have moved one mailbox from the 2010 to 2016.  Messages from outside the organization are still routed through the 2010 servers.  Initially I was ablel to send messages from outside to the mailbox on 2016.  Currently the messages for the 2016 mailbox are getting stuck on the 2010 server.   I can see them in the queue viewer.  They are in the next hop domain "hub version 15" with the last error "45 4.4.0 Primary target IP address responded with: "451 5.7.3 Cannot achieve Exchange Server authentication."

I believe this started when I created SMTP relay receive connectors on the 2016 boxes.  Initially those SMTP relay connectors included the subnet the Exchange 2010 servers are in, 172.16.4.0/24.  I have since removed that subnet from the 2016 SMTP relay recieve connectors but the messages are still stuck on the 2010 servers.  

I have verified the 2016 servers each have a HubTransport receive connector with Exchange server authentication, Exchange server, and Legacy Exchange servers selected.  The scope is unchanged to allow all IPv4 and V6 addresses.

I have enabled logging on all the receive connectors and have checked the log file in C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\Hub\ProtocolLog\SmtpReceive for errors.  It's a large file and a bit cryptic but I don't see any errors or references to the 2010 servers.

I also checked the log on the 2010 servers located in D:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\Logs\ProtocolLog\SmtpSend.  I find no reference to the 2016 servers in these log files.


What am I missing?

Thanks

Bill