If your father created the email account and gave it to you without having "gifted" it he would have retained the ability to manage it and access it because it is still one of his email accounts.
He would, when setting it up have also set up a password and you would not have been able to change it on your own. Only the "master" account holder would have been able to do that.
He would have been able to access the email account through his MyBT or by directly logging on to it or by logging onto his own "master" email account and using a drop down box that would show all the email accounts that were still under his management. The drop down facility has since been removed and is no longer available.
If he "gifted" the email account to you you would have had to "accept" the email account and set up a BTID so that you could manage the email account. This would have allowed you to change the password without the "master" account holder's intervention. He would not have been able to access the email account through any route other than if he knew the password.
Once an email account is "gifted" the "master" account holder loses all management rights except the ability to delete the email account.
See link about "gifted" email accounts. The BTMail system has been changed over the years so some of the things mentioned in the link are only relevant to today's system and were not necessarily the same in 2014 although the gifting process was the same.
Creating BT Email addresses? | BT Help
