Check that you have not blocked either their email addresses or the domain of their email.
To do that 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 domain and/or the email address you want to unblock and select it. Click on "delete".
