I have an Exchange 2007 Server with all the updates and service packs installed. I have just installed Exchange 2013 CU1. I created a test mailbox on Exchange 2007 and migrated over to the 2013 server. When I try to send an internal email from the Exchange 2013 mailbox to a Exchange 2007 mailbox I get the following error and the recipient never receives the email:
Remote Server at returned '451 4.4.0 Primary target IP address responded with: "451 5.7.3 Cannot achieve Exchange Server authentication." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.
Authentication settings on the send connector is configured to allow Exchange Authentication. Any assistance would be appreciated. Thanks!