I did report the account details issue to BA and received a response yesterday asking for more information. I responded to the email and delivery of the email failed. I tried again this morning and same error message 🤷🏻‍♂️

Delivery has failed to these recipients or groups:

British Airways Executive Club ([email protected])
The recipient's mailbox is full and can't accept messages now. Please try resending your message later, or contact the recipient directly.

I did use the web form to send my response but this is really poor.