The mail could not be delivered to the recipient because the domain is not reachable. Please check the domain and try again

After replying to another thread (http://answers.microsoft.com/thread/dd8811a0-c873-4e37-9d1c-da69a58f6a15), and providing more information in private messages to Microsoft support, I was asked by Eugene R. (http://answers.microsoft.com/en-us/profile/9e318ed7-b793-4987-af5d-16dce522e79e) to open up a new thread so this issue may be escalated.

To summarize, we are unable to receive messages on our domain (xdrradiology.com) from any Microsoft mail service (Hotmail, MSN, outlook.com).

We have no trouble receiving mail from other sources (including gmail.com for example). The problem is specific to email being sent from MS mail services.

Here is an example of the error (I put asterix in to mask part of the email address - I can send in the full header if you'd like in a private message):

Final-Recipient: rfc822;********@xdrradiology.com
Action: failed
Status: 5.5.0
Diagnostic-Code: smtp;554 The mail could not be delivered to the recipient because the domain is not reachable. Please check the domain and try again

This is preventing certain customers from reaching us so we'd like to get this resolved as soon as possible. I already had our web host (hostingdivision.com) look into things on their end, and they are not seeing anything unusual. And, like a said earlier, the addresses on xdrradiology.com are not having any trouble receiving mail from other sources. We are also not having any trouble sending mail to any sources from xdrradiology.com addresses.

Thank you very much for your help!