Exchange 2010 /2013 Queue 451 4.4.0 Primary target IP address responded with: “451 5.7.3 Cannot achieve Exchange Server authentication

I came across the issue above recently at a clients. I was installing exchange 2010 into an exchange 2003 organization everything was going well , users where migrating fine. I had created the routing group connector between Exchange 2010 and Exchange 2003. I thought all was well, mail was flowing out of the company fine, and fine flowing back in. As users moved from the exchange 2003 store to the exchange 2010 store obviously people where emailing internally while being on exchange 2010 to people still waiting to be migrated to exchange 2010.

I started getting complaints that mail was going missing. I checked the mail queues to find the error below.

Solution

Turns out that for some reason Exchange 2010 could not log into Exchange 2003. So off I went on to the Exchange 2003 server.

I expanded the Admin groups , then to the 2003 server. I then expanded to protocols, SMTP and right clicked SMTP virtual server and selected properties.I then select Access and authentication.

 

I noticed that authentication was only set for anonymous. This was why Exchange 2010 could not log into Exchange 2003.

I quickly added ticks to both Basic Authentication and Integrated Windows Authentication then clicked OK

 

To make these changes come into effect immediately I had to ..

Restart the SMTP services on the Exchange 2003 server and run the following command on the Exchange 2003.

 iisreset

Then on the Exchange 2010 server I restarted the Exchange 2010 transport service.

I then checked my queus and mail was now flowing freely.