Outlook Express has a nasty habit of 'losing' the settings it requires to connect you to the outgoing mail server. (To be be fair to Microsoft, your email scanning software has to temporarily change your server settings when it scans incoming and outgoing mail. Sometimes the two different pieces of software fail to communicate properly, leaving the settings 'screwed up').
First, make sure you know the address of your email provider's SMTP server. The information will appear somewhere on the support pages of that provider. However, if your email service is provided by a main UK ISP, it will be quicker to look here for the info:
http://www.kitz.co.uk/isp/dns.htm
Then open Outlook Express. Go to Tools > Accounts > Mail.
If you've only got one email account, it will probably already be selected. Otherwise click on the relevant account, to highlight it.
Click 'Properties' and 'Servers'.
Take a look at the information in the SMTP server setting. It will probably be incorrect. If so, enter the correct information. Click 'Apply', 'OK' and 'Close'.
Chris