Hi Latedev 👋.
Thanks for the update on this, with Legacy emails such as Blueyonder, NTLWorld, Virgin.net etc, given their age and changes in security and GDPR they are prone to be seen as emails that have been "left" by their owners and used for potential spam, cloning, virus delivery and more, due to this we have asked customers who wish to continue to use this to have a secondary back up that is a provided by an email domain such as Gmail, Outlook etc as not only to recover access but to ensure its security of your email address.
I hope this helps.
Sabrina