Hmm,
Chuck wrote "you must always use your ISP's outgoing mail server for sending". That's certainly not true, since I send mail directly through the outgoing server of a 'non-ISP' server (Gawab).
However, I agree that the problem lies with your SMTP server setting. Both Outlook and Outlook Express have a nasty habit of 'losing' mail settings. To be fair to Microsoft, it's not always the fault of their programs. Anti-virus software makes temporary changes to these settings, to enable it to scan mail. Sometimes the security program and the email client fail to communicate properly, and the information becomes scrambled.
First, check that you know the address of the relevant SMTP server. For TalkTalk, it's smtp.talktalk.net.
Then go to Tools > Email Accounts.
Check that the relevant account is highlighted. Select 'Properties' and 'Servers'. Check that the correct address is entered in the SMTP field. If not, correct it.
Sometimes, Outlook will display the correct SMTP address but still fail to use it. If so, the simplest solution is to delete your email account (Tools > Email Accounts > Remove) and create it again using the wizard which sets up new mail accounts (Tools > Email accounts > New). Just ensure that you know your username, password, SMTP and POP3 addresses before doing so. (For TalkTalk, the POP3 server is listed differently on various sites. It's possible that all of these will work: pop3.talktalk.net, pop.talktalk.net, mail.talktalk.net. However, it would be sensible to check the address used by your existing account - under the 'Servers' tab - and use that one).
Chris