The new BT Mail appears to either not know, or like a bored postman at the sorting office, slings the odd one or two in the wrong bins for no reason.
On BT Yahoo, spam was, for me, quite well sorted, I very rarely had ' proper ' mail delivered to the spam box and vice versa.
Since being migrated I've been getting ' proper ' mail dumped in the spam box, nor is it consistent.
For example, I subscribe to Amazon and Ebuyer email offers.
Sometimes they land in my inbox, sometimes in my spam box.
I had one from each yesterday in my inbox, as they should be, yet one from each today delivered to my spam box.
Ahhh I thought, having both days emails I'll compare full headers to see if there are any clues why this should happen......... ohhhhhh forgot, this isn't available on Bt Mail.
Off to Thunderbird, checked through the headers and could find no reason why todays went in the spam box.
Could it be because it's Friday and the server wants an early finish. 
