I just installed Exchange 2010 and have moved mailboxes, got all of my certificates working correctly. But one issue has popped up and I can get a handle on it. Some external sites are not accepting our email and is sending NDR's back to the user. I am not seeing any error message, but it just says that the message expired. The interesting part is that this is only happening with certain sites and until those users contact me, I don't know which sites.
How can I fix this issue?
I wanted to add that I have 2 Exchange Servers with 2 different external IP's in my DNS. I MX record so both servers can go through our Barracuda for email delivery.