Are you presently a BT Broadband customer or are you paying for your email through a BT Premium email account?
You never said in your post so I have assumed that you are using the email app on your iPad if you were able to delete the email account and re add it.
The best advice when your problem happens is to do nothing, especially do not change passwords or delete the account from your device as that can cause more problems than it solves.
The first thing to do when it happens is check to see if you can log on to your email account via a web browser. If you can there is obviously nothing wrong with your password or account hence the advice to do nothing.
Did you try that and were you able to access your email account and send/receive emails?
If it is an intermittent it is most likely a BTMail server issue.
Did you try turning your BT Hub off for half an hour? When you turned it back on it would allow your connection to be issued with a new IP address and connected to a different email server.
