I'm not on BT but NTL & ISDN
You'll find that most UK ISP's dont support OL, OE only, I believe it simply
a case of OL being more than just an email app, therefore requiring more
support.
I'm assuming you have tried OE with your BT account? - dont worry about
downloading mail you can allways export the mail to OL once its OK, if you
havent it might be an idea to do so, just to double check your settings.
If you have done the above and OE is OK, then;
In the OL accounts, accessible via the Control Panel>Mail Applet under
advanced for the account check that the Connection is set correctly.(check
with OL closed)
If so run the test if it reports the same error.
1) run detect/repair from your office cd, or within OL, then close OL
2) Mail applet, create a new profile, prompt for profile
3) Add a new data file, if any existing data files are shown remove these
4) Start OL,under this profile, File>Import>mail settings from OE
Then check again, (presumably you know you can send a mail to yourself)
Should keep you occupied for a while, see how you go