I am not receiving emails from known contacts periodically, although they say they have sent them and not 'bounced' back not delivered.
Make sure that their email address or their email domain, the part after @ is not in your blocked senders list.
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".
Done all that, sometimes I receive emails ok then other times non-receipt. This is people in my address book.
One other is from a NHS address who is trying to email.