I'm a bit confused how the mail is sent/received,admin notification. This is how i see it, please correct me if I am wrong. A member registers and the mail is sent via the host with a validation email. The return validation email is then sent (again)via the host to the the Admin, also the notification of a new member is sent the same way. In other words the hosts Email sender controls all emails, except for the recipients mail client.
After some extensive testing with the support staff I received this support email.
I agree with most of your assessment above, but surely the hosts ISP's mail server is out of my hands. As for IPB Standard template I'm certain there is nothing I can change there. I do mark every "spam mail" as authenticate and hopefully that helps. I have run many forums, and only once had a similar problem. It turns out that the hosts server was rejecting the notification as spam as my admin address was not actually being shown within the cpanel. By adding an account into the cpanel mail system admin@mydomain did it solve the problem. I have never experienced any problems before. Maybe it's the answer. If my assumptions are wrong then please let me know. Thanks....