Outlook Express has a nasty habit of screwing up server settings, which can lead to the type of problem you're experiencing. (To be fair to Microsoft, the problem isn't necessarily directly related to any fault with Outlook Express. When you send and receive email, your antivirus program has to temporarily change the server settings, to enable it to scan your mail. Sometimes the two programs fail to communicate properly, resulting in erroneous server settings).
First, make sure that you know the POP3 and SMTP server addresses used by your ISP (or whatever other email service you're using). If you're getting your email from one of the main UK ISPs, they can be found here:
http://www.idl.uk.com/email/isp_settings.htm
(Otherwise, check the support pages of your email provider).
Open Outlook Express. 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 'Properties'. Check that the information under the General tab is OK. (Amend if necessary). Click on 'Servers'. You'll probably find an error under that tab. Correct it. Click 'Apply'. Click 'OK' and then 'Close'.
That usually fixes the problem but Outlook Express sometimes displays the correct information, under the General and Servers tabs, while actually using incorrect data called from the registry. If so, start as above but, instead of clicking 'Properties', click 'Remove' and 'Yes'. Then click Add > Mail, and re-create your account by following the instructions.
Chris