I've had this in the past, a number of things have caused it.
Firstly double check your outgoing mail server settings - am sure you have already.
Secondly if you, like me, use Outlook Express for multiple email accounts, although it will pick up mail from all , it will only send from the relevant ISP - - for example I sign on via ntl and use OE for btopenworld, tiscali and ntl and have configured to collect all. However, I can only send from ntl account.
One more thing, I have also had troubles with this when ntl have had trouble with their mail server - - -it rights itself eventually.