Windows Mail Can Reply But Not Send New Email

I'm running Vista and using Windows Mail. I can recieve emails no problem and when I reply to an email it gets the recipient fine. However, if I create a new email and send it goes out of my outbox but the receipient never recieves it. I have spoken to LCN who host my email and they have run tests to hotmail, gmail and business accounts. They say the email get through to the recipient server fine but they cannot track them from there. Trying this myself I can confirm that there are not getting through to regular contacts inboxes at business and webmail accounts and they are not even going to junk mail.

They said to try changing the outgoing to my ISP SMTP details which I did but still no joy. They and my ISP have both said this is all they can do. I've turned off my anti-virus can for outgoing mails as well but still have the problem. Everything was working fine till a few days ago. When I called LCN they said my nameserver needed updating, which they did, and that it would take 24 hrs for the DNS propagation. It's now been 3 days and still the same problem. Email host and ISP say they can't help. I've checked and I'm not on a blacklist.