Hi Ken,
BT business (btconnect.com) is our partner. They're upgrading BT Business email customers to Office 365.
Since your mail service is hosted in BT business, some settings of your account are managed by them. About this issue, I suggest you contact the BT support. They’ll be more proper and professional to resolve this issue. In the above posts, we haven’t realized this, I am sorry for the careless. Thanks for your understanding.
Best regards,
Young Yang