The entered e-mail domain has no valid MX record
I re-validated our settings, check the various support forums and even upgraded the forum 1 minor version. None of this helped.
As per PHPBB support forums, I have (at least for now) simply disabled that check, and confirmed that registration is now possible.
Again, I am sorry for any problems this may have caused. PHPBB is great when it works but it can demand alot of upkeep.
Have a great day!