Exchange 2010 Error 451 4.4.0 Error DNS Query Failed

When logging on to the Exchange Server and looking at the Outbound queues, we noticed mail for only a particular domain being held with the 451 4.4.0 Error DNS Query Failed error. Other symptoms:

1. The nslookup command run on the Exchange server could resolve the domain, proving the internal DNS server was normal.
2. After gaining the MX record from who.is, the nslookup command resolved the mail server’s IPv4 address, proving the receiver’s mail server was resolvable.
3. A telnet session to the MX record successfully contacted the suspected domain’s mail server on port 25.
4. Internal email delivery was functioning.
5. All other external mail delivery was functioning.

Now we’re down to Exchange itself, since everything DNS related on the server is working correctly. This is the current fix we use:

Step 1: Configure and External DNS server
1.Click Server Configuration
2.Right click the server and choose properties
3.Click The External DNS Lookup tab
     1.Choose Use these DNS servers
     2.Add the DNS Server IPv4 address
     3.Click Apply

Step 2: Configure the Hub Transport to use the External DNS for external domains.
1.Click on the Hub Transport of Organization Configuration
2.Choose Send Connectors
3.Right click the connector and choose properties
4.Click the Network Tab
5.Check the Use External DNS Lookup… box

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

After the Configuration, Restart the Transport Service. The queue should empty immediately.