We do not recommend that you rely upon an email address linked to a specific ISP – for example, a btinternet.co.uk address.
We don’t provide an email service as there are many good – usually free – email services available. Google (Gmail & Googlemail), Microsoft (Hotmail & Outlook), GMX, and other providers operate such services. We use them for our personal email accounts. Or, you can choose to purchase your own domain and an email service linked to that domain. (BordersOnline can assist with business grade email services)
You should consider switching your email address from your previous internet service provider account such as BT to another service as soon as possible. However, you may be able to retain your email service linked for a former ISP provided that you are willing to pay for it. That seems to be the situation for btinternet.co.uk accounts.