I regularly receive messages from the company 'Uniqlo' and they always appear in my spam folder. I have added their address as a genuine contact but they still go in the spam folder. Why doesn't BT Mail recognise a genuine address when it has been confirmed several times that it is an address that I recognise?
If you are marking the emails as not spam the spam filters should eventually catch up.
Adding an email address as a contact will not always stop emails from them going into spam.
If you have not already done so you can add the domain that the sender is using to your "safe senders" list that can be found by going into your email account settings then going "Mail" > "Safe senders". This should cover all email coming from that domain rather than just from one email address.