Trying deleting the account and then creating it again. (It often fixes problems with Outlook Express. It's perfectly safe to do it; you won't lose any existing mail in your Inbox or other folders). To do so, you'll need to know the SMTP and POP3 addresses for your mail servers. If you don't know them you can find them on your ISP's support pages (assuming, of course, that you get your mail through an account linked to your ISP). Alternatively you can probably find them here:
http://www.kitz.co.uk/isp/dns.htm
or here:
http://www.idl.uk.com/email/isp_settings.htm
Click on Tools>Accounts>Mail and then on the name of your account. Click 'Remove' and 'Yes'. Then click on Add>Mail and create the account from scratch.
Chris