The domain, which is the part after @ such as @btinternet.com is unique to each email supplier. You can not transfer an email address to another domain. ie you can not have joe.bloggs@gmail transferred to become joe.bloggs@btinternet.com
If you want to be able to access all your present email addresses ie your BT email addresses and the other email addresses that you presently have, can be set up to be accessed from one email client/app or your BTMail webmail page.
This however would require you NOT to close the email account with the other company, which if you did would mean you would lose access to that email address and all your emails from that account.
