When a user sends e-mail messages to a remote domain, some e-mail messages are queued on an Exchange Server Hub Transport server or Edge server

Symptoms

You have a Windows Server 2008-based computer that is running Microsoft Exchange Server 2010 or Microsoft Exchange Server 2007. Additionally, you have the Hub Transport server role or the Edge server role installed. On this computer, you may find that e-mail messages that are sent to some remote domains are queued as "retryable." In addition, the Last Error on the queued items is shown "451 4.4.0 DNS query failed".

Cause

This problem occurs when the Exchange Server server tries to query AAAA records for the remote domain from a Domain Name System (DNS) server. However, either some DNS servers ignore the AAAA query or they return an unexpected response. This problem occurs because Exchange Server 2010 and Exchange Server 2007 do not use MX or A records.

Note The AAAA resource record type is a new record that is specific to the Internet class. Additionally, the AAAA resource is used to store a single IPv6 address.

Workaround

To work around this issue, create Send connectors for the affected remote domains. Then, configure the Send connectors to use a smart host for remote MX records.

Resolution

Exchange Server 2010

To resolve this problem in Exchange Server 2010, follow these steps:

1. Modify the send connector to use External DNS.
   
   

   Set-SendConnector <connector name> -UseExternalDNSServersEnabled $true

2. Ensure that DnsFaultTolerance is set to Lenient in the EdgeTransport.Exe.Config. For more information about this problem, visit the following Microsoft Web site: 
   
   General information about DNS Query Failure Sensitivity in Exchange 2007 SP1 and SP2

Exchange Server 2007

To resolve this problem in Exchange Server 2007, follow the following steps:

1. Install the following update rollup:
   

   972076 Description of Update Rollup 2 for Exchange Server 2007 Service Pack 2
   
   

2. Modify the send connector to use External DNS.
   

   Set-SendConnector <connector name> -UseExternalDNSServersEnabled $true

3. Ensure that DnsFaultTolerance is set to Lenient in the EdgeTransport.Exe.Config. For more information about this problem, visit the following Microsoft Web site:
   
   

   General information about DNS Query Failure Sensitivity in Exchange 2007 SP1 and SP2

Status

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.