Okay.
No confusion.
Just because you have the same domain does not mean you have the same email server nor could the email server setup be the same as before or use the same software for the email server.
If it were me I would test the main email account used in ubb outside of UBB first.
Just to make sure the email account works both ways.Send and receive. Before looking at ubb as a culprit.
It could be either one but you need to start somewhere.
Also you could open a support ticket with UBB.
