You have possibly blocked it in error. You can check and unblock it.
Log onto your BTMail account using a browser, not an email client/app, and then click on your username at the right hand side. Select "Settings" then go to the left hand side and select "Mail". Scroll down and click on "Blocked Senders". Scroll down the list until you find the one you want to unblock and select it. Click on "delete".
You should also add the senders's email address to your "Safe Senders" list which you can find beside the "Blocked Senders".