Check your blocked senders list. Check if the email address or its domain, the part after the @, is there.
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".
