Question : 451 4.4.0 Primary target IP address responded with: ""554 mail.domain.gov""

451 4.4.0 Primary target IP address responded with: ""554 mail.domain.gov."" Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.

Answer : 451 4.4.0 Primary target IP address responded with: ""554 mail.domain.gov""

Have you tried disconnecting all HD's except the one that has the OS?