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