[Rob]

Your IP suggests you are using a BT connection. Are you sending email through a BT address or something else?

Most email can only be sent to a server hosted by the ISP you are connected to. Thus if you are on a virgin media (ex ntl) service, you have to send your email via the smtp.ntlworld.com server. However if you are using your PC on a different ISPs service, and trying to send an email using your virgin media account, you have to use the other ISPs mail server to send, as Virgin Media (and most ISP) as an antispam measure, only accept mail from inside their network.