Unfortunately there is not much else you can do. For as long as I can remember, the BT handling of what it does or does not regard as spam has been problematic.
The only true solution is to use Gmail for important emails.
That's what I've been doing with the ones that are important but it's so annoying having to do it all the time I mark them as safe, move to inbox then they reappear in spam!
If the emails are coming from the senders own domain, not a main stream domain (the part after @) you can add the domain to your "Safe Senders" list as well as the email address. It might help but a@licquorice has said, BT's spam system and how it "works" is a mystery.
I find that setting rules to send emails to your inbox (or spam for that matter) are more effective than either the safe senders or blocked senders lists.
