While migrating from exchange 2003 to exchange 2010 you see mail queus building up between the exchange 2010 server send mail to exchange 2003. The error yuo see is Exchange 2010 Queue 451 4.4.0 Primary target IP address responded with: “451 5.7.3 Cannot achieve Exchange Server authentication. Solution here.
We need to use the set-emailaddresspolicy cmdlet to set the the exchange 2010 server to be able to manage the recipient policy, run the command in this article to fix this issue. The specified e-mail address policy couldn’t be edited. E-mail address policies created with legacy versions of Exchange must be upgraded using the ‘Set-EmailAddressPolicy’ task, with the Exchange 2010 Recipient Filter specified