When an outside sender sends an email using a distribution group that has a user inside our organization as a recipient then the email is bounced just for our user back to the sender. We use MX Logic as our hosted email, I have checked the logs on MX Logic and it shows the following error: "Detail: first attempt failure: 451 unknown error writing data to backend for message"
Here is the part I don't understand, when the email is sent directly to our user it goes through fine. I've checked the SMTP logs (turned on logging) and they do not show the message being blocked. Just the message that is going through. I've contacted MX Logic and they said that our Exchange server is blocking the message but I don't see any record of it using message tracking nor going through the SMTP logs. Anyone have any other suggestions for trying to track down why this message will not go through?
Server 2003 R2 x64
Exchange 2007 SP2