I have a few places where email is part of a login, I will unlikely pin them all down, is there a grace period when you leave BT?
It is an inderminate period of time but at least 60 days. However, if the email address is simply being used as a login to a site (rather than receiving correspondence from the site) it is just a string of characters that happens to be an email address and it being deleted as an email address by BT will have no effect on your logins.