I'm just looking for validation here more than anything else.
I have recently moved offices and as part of that we started using a new ISP, who gave us a new range of Public IP's. External DNS has been updated and my Hub Transport server is receiving emails fine internally and externally.
When however we send them externally I get the above NDR. We use Maildefender as a smart host and so I'm wondering if its just a case that they are blocking the emails based on the fact that they don't have details of our new Public IP address?
Or is there anything else that I need to check before going to them?
I will post the NDR in the next message for people to cast judgement.