I'm currently having an issue where our exchange 2010 server is no longer sending emails to our Ex03 users. We currently changed the primary domain for email accounts on both Ex2010 & Ex2003 environments. When I send an email to a test account in my Ex2003 environment from a Ex2010 user account I get the following error from my delivery report tool:
3/12/2015 11:59 AM Ex2010.domain.net
The e-mail address for recipient "email@example.com" was updated to the e-mail address "firstname.lastname@example.org". The message is in the process of being delivered."
"The last attempt to send the message was at 3/12/2015 12:12:10 PM (UTC-05:00) Eastern Time (US & Canada) and generated the error '451 4.4.0 Primary target IP address responded with: "535 5.7.3 Authentication unsuccessful.." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.'."