I have been receiving this message for over a week. "We are experiencing issues with the service, please try again later." I can access email on my iPad and Android phone.
Is this happening before or after you have tried to log on?
Try using this log on page. BTMail
What browser are you using to access your email?
Try clearing your browser's history and cookies.
Try a different browser.
Try turning your BT hub off for about half an hour and when you turn it back on it should connect to a different email server.