I had the same problem a few months back. At the time I was with Orange but have since moved to BT.
I always dowloaded my emails using OE and did not actually log on to the actual Orange site. After a few days of painfully slow email downloads I decided to retrieve them from the website.
I could not believe what I saw. There were over 14,000 spam mails in my junk folder and OE had slowed because it was scanning each message before sending any new mail to my inbox.
I ditched the junk and Orange.
It's a long-shot as BT are pretty good at filtering spam but worth a try.