Hi, We have had trouble sending email to one of our customers for a long time. I finally got a SPF record put in yesterday and was hoping this would make them trust messages coming from our SBS2008 box running Exchange 2007 finally. We sent out a test message late yesterday and the queue viewer shows it in there with a: 451 4.4.0 DNS query failed. Properties of the message show last error is empty. Is this because it hasn't gotten past the DNS query to even try to send the message? If I do a nslookup -q=mx thedomain.com on the server (with my customers domain substituted), it resolves ok. Any ideas? Thanks, Alan

What’s the exchange version? Have you seen KB 972744? Can you can telnet the unreachable remote domains to send the test message on the exchange internet-facing transport server? Please refer to suggestion in this article for troubleshooting Please enable the protocol logging on the send connector, Connectivity Logging on the exchange server and then reproduce the issue, which will give you more information Please check the network trace of undelivered mail flow on the transport server: a. Please start Network Monitor on transport server to capture the network trace b. Send test mail and reproduce the issue c. Stop the network monitor, and save the trace d. Add the affected domain’s info to host file e. Start the network monitor and send the second test mail f. Stop the network monitor, and save the trace After capture the traces, please see if the following traces exist: ============ Standard query MX ExternalDomain Standard query response MX ... Standard query AAAA ExternalDomain Standard query response, Server failure Standard query AAAA ExternalDomain Standard query AAAA ExternalDomain Standard query response, Server failure ============ Resources: How to capture network traffic with Network Monitor Notes: This forum is for the exchange issue on the windows server environment. For the exchange issues on the SBS server, please use the “Small Business Server” forum which would b the best place for seeking the suggestionsJames Luo TechNet Subscriber Support (http://technet.microsoft.com/en-us/subscriptions/ms788697.aspx) If you have any feedback on our support, please contact tngfb@microsoft.com