I do it was a internal DNS issue in my case. At that time I was using same domain name for both internal/external and although it worked with my Att router originally my assumption is it broke when I migrated to the MX. I've since moved to split brain DNS for my AD and also bought a public IP block so I use public IP on the MX outside interface essentially removing MX from being behind NAT device and allowing NAT at the MX
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