I'd be prepared to bet that simply deleting your email account, and then re-creating it, will fix the problem. (Deleting the account only removes the connection settings. Any existing mail in your Inbox, or elsewhere, won't be affected).
Go to Tools > Accounts. Click on the 'Mail' tab. If you've only got one email account it will probably already be highlighted. Otherwise click on the relevant account to highlight it. Click on 'Remove'. Click 'Yes'.
Now click on Add > Mail. Then follow the instructions (starting from Step 4), here:
http://www.virginmedia.com/help/email/settingu poutlookexpress6.php
(NB: After Step 5, you need to click the 'Next' button, at the foot of the page, for the rest of the instructions).
Chris