It seems that iOS mail app does not work with the UNT servers. This seems to be an overgeneralization. I contacted UNT helpdesk for assistance. I set up my account correctly and then it downloaded one message which stated that the service was blocked on my device. So account creation was successful, but there is something on their end... The UNT help desk was unhelpful in that they couldn't tell me anything except that "sometimes the mail app doesn't work with our servers, we recommend using the outlook app". Well it does work, else I wouldn't have successfully downloaded the email message saying I was blocked...

The best guess I can come up with is that older versions of mail app in iOS are blocked because they have a security flaw in them or UNT has middleware deployed on its ActiveSync connections.