After configuring hybrid for an Exchange 2010 environment, I tested mail flow from a on premise mailbox to an Exchange Online mailbox. However, the messages were queuing with the ‘451 4.4.0 DNS Query Failed’ message.
The traditional resolution is to set the “Outbound to Office 365” send connector to “Use the External DNS Lookup settings on the transport server“. Yet still the messages continued to fill the queue.
Looking into the transport config settings of the only hub transport server, I noticed something odd. There was no value set for the “External DNS Lookups”…
I proceeded to set “Use network card DNS settings:” to ‘(All Available IPv4)’ and restarted the transport service. After the restart, queued mail items were successfully delivered to the intended Exchange Online mailboxes.