We migrate from Mirapoint to Exchange 2010 last week.
Everything went smooth until yesterday a few users complaint that they are not able to send email to certain domain.
The error differ for every domain name.
The first error (eg firstname.lastname@example.org):
451 4.4.0 Primary target IP address responded with: "421 4.2.1 Unable to connect." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.
The second error (eg email@example.com):
451 4.4.0 DNS Query failed