I'm sorry to say that it's not just BT. AOL are also doing this and I believe Freeserve, and maybe others as well. I don't know why they are doing this, except for the reasons I have read about so far.
It is alleged that the major ISP's are at times blocking large amounts of incoming mail in an attempt to deal with "Spam". Just how they come to use this method of dealing with it, is beyond me. Regards. j.