I've been a BT customer for many years and still use my original email address - the format is email@example.com. I'm aware that as a result of changes to the BT mail service over the years my email address is actually firstname.lastname@example.org and I can log on to my email account using the .t21 address.
When I log on to my billing account using my BT ID I can see that the email address email@example.com is one of the email addresses that belong to this billing account. However, the address firstname.lastname@example.org is not shown on the list of email addresses that belong to me, and if I try to link it to my billing account by importing it I can see that it is linked to a different BT ID - not mine.
Am I at risk of losing the address email@example.com as it is linked to someone else's billing account? Or should I not worry, because firstname.lastname@example.org is not actually an email account but is only an alias for email@example.com?
I doubt that any of the forum users will be able to assist/answer your question so I have notified the moderators of the forum about your problem. Once they have read this they may be able to help. They are very busy at present so it can take up to 24 hours for them to contact you. They will do this by posting on this thread.