'Sticking' emails are quite a common problem. Fortunately, most ISPs now allow you to access your e-mail through your web browser, as well as via a POP3 client (such as Outlook). I'm reasonably confident that BT Internet allows you to do this.
So, go to the BT Internet page and log in. Then click to view your mail online. Download the big file to your PC (unless, of course, it's obviously junk mail!) and then delete it from the server. Log out and try accessing your mail via Outlook. Hopefully, your mail should now arrive normally. (If not, return to the web access and, after reading them and/or saving them to another folder, delete the remaining emails. Then send a message to yourself and, after logging off from the web-based access, see if you can retrieve it via Outlook).
Chris