The only way i can think is, currently you have the bt email setup in Outlook as default also when you reply to the email in the top corner there is a button that says accounts, click it and it should show a drop down list and select that email you want it to show up as.
Do you have it setup as a real email not as a forwarder, so you can check your emails via
http://www.yoursite.url:2095 or do you have it set as, when you get a email to
email@domain.com it gets sent to another email such as your personal one.
Thanks,
Hope this helped
Gavin Brydon