Did anyone ever make headway on this issue? We’re experiencing the same thing, although UPS seems to work fine in the same system. Even a way to get more detailed information on why the attempt was rejected and/or what data was included in the attempt would be helpful.
In the event someone else has this problem in the future: we never actually found the cause for this, but deleting and re-creating the connection to EasyPost in Business Central appears to have resolved it. Your mileage may vary, obviously, but it’s worth a try.