Here is a good one! The president of the company comes to me with an odd mail issue. I have since found it’s not that odd, just one I had not personally experienced before. Allow me to take you on the journey to troubleshoot and solve this..
    "Chad, I can’t seem to send mail to this one person. Yet they can send mail to me from scratch, just I cannot reply"
What do you begin to think right off the bat? Not our servers, they are stable as can be! Well my first instinct was that it was likely an issue on their end. I had dropped a mail message (From my personal acct obviously) to the point of contact there asking if their mail admin could look at some SMTP logs to confirm or deny our servers are even talking. My through process to this point is – firgure out what is or is not working to eliminate either their systems or ours.
to start the mass elimination process i then sent another few test mails from different POP and web based mail systems to their servers. So we know now that their system can accept incoming SMTP connections from multiple sources inluding new domains. Now I started to think it was an issue with my server. No users from my system could send mail to theirs, but they could send to us. That tells me that their servers can establish an SMTP session with mine. from there i hit up the Exchange server 2007 toolbox. I brought up the queue viewer and noted the namespace i was trying to send to and it had a retry status with an error of "451 4.4.0 DNS Query failed." I then attacked NSLOOKUP to see if i could resolve their domain name for MX and A records for the SMTP hosts. I was able to do this with no issues.
So this tells me outbound mail isn’t even hitting the submission queue on this local server to even be sent. this makes the mailflow troubleshooter useless to me. What shocked me was that it was still only one domain having this issue. If I had DNS issues, wouldn’t it affect ALL external namespaces? not so. the kicker is in HOW the external namespace is being resolved. Remember your exchange box REQUIRES IPv6 for installation and will try to resolve all domain names for sending and categorization by all means (IP Stacks) neccessary.  Here is where i started doing some research and found some nice posts / articles…
Technet mail transport troubleshooting document  – i know it says Edge Transport , but it can be used for Hub Transport too
The article i got my final answer from  – Scroll down to the bottom in the "Troubleshooting DNS" section
So my final fix was adjusting the organization’s single send connector. On the properties dialog box and Network tab, It was set to just "Use domain name system (DNS) "MX" records to route mail automatically". At the bottom of this page there is a check box for "Use the external DNS lookup settings on the transport server". Once i checked this and restarted the Microsoft Exchange Transport service the mail flew out of the queue when i restarted it :-)

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