Thanks for that Ann. (I hate writing 'Are you sure you really meant that' posts; people tend to get offended!).
I'll try to answer point by point:
1. As a short-term workaround, remember that you can access your email without using Outlook Express by logging into TalkTalk's webmail service here:
http://www.talktalk.co.uk/mail/?check_cookie=1
2. As Spikey has pointed out, some email problems can be related to temporary problems with the sending server. (i.e. TalkTalk might be experiencing problems). So simply waiting a day or two (and using webmail, as above, in the meantime) might be all that you need to do.
3. If the problem doesn't clear itself, try checking the settings for your account in Outlook Express, as follows:
Go to Tools > Accounts > Mail. If your email account isn't already highlighted, click on it to make it so. Click 'Properties'. Look through what's there, taking particular note of what you find from the 'Servers' tab, next to 'Incoming mail'. It should match what you find from here:
https://help2.talktalk.co.uk/email-settings-imap-pop3
If it's incorrect, change it, click 'Apply' and then 'OK'
However Outlook Express has a nasty habit of showing the correct details under 'Properties' but not actually using them. If you find nothing wrong, start as above but, instead of selecting 'Properties', choose 'Remove'. (Don't worry. You won't lose any emails; it's only the account settings that are removed). Then use 'Add' to create the account again, using the information from my TalkTalk link above.
4. Despite its propensity to 'lose' account settings, there's not really much wrong with Outlook Express. (If you want something better, try Thunderbird:
http://www.mozilla.org/en-GB/thunderbird/ ). It's Internet Explorer 6 that I was describing as 'antiquated'. Get a decent web browser!
http://www.mozilla.org/en-GB/firefox/new/