I am assuming that you have set up Outlook as an IMAP account.
The 11,000 emails must be on the BT servers and Outlook is trying to download them in order to synchronise with the servers because that is what an email client has to do if it is to do its job.
The only way to stop Outlook from downloading them is to close Outlook then delete the emails from the BT servers by logging onto your BTMail email account using a web browser. However doing that will be a long tedious job as there is not a delete all button on web mail and it is only possible to delete about 200 emails at a time.
You would be better to just let the emails download even if that means leaving it to run over night. Once they have downloaded onto Outlook you will be able to delete them en masse if you want to still do that as Outlook has the ability to do that. This will also delete them from the BT servers so make sure you are only deleting the ones that you no longer want.
