It's a standard feature, with almost all ISPs, that you can connect to their POP3 (incoming) server via any internet connection but you can only access their SMTP (outgoing server) when using their own service to connect to the internet. (It's a deliberate policy to prevent people, who don't pay a subscription to the ISP, from using their e-mail service ). i.e. you'll only be able to send mail through your NTLWorld/Virgin account when connected to the internet with Virgin.
If you wish to continue using your NTLWorld email account, the only way to access it, when connected to the internet via a different ISP, is to use the webmail facility, here:
http://webmail.ntlworld.com/
Much better would be to switch to an email service which is independent of any ISP but still allows access by both webmail and an email client such as Outlook Express. The most popular service is GMail but I prefer Gawab. Details of how to set up a free Gawab account are in my post here. (Paragraph 3, et seq):
http://www.theanswerbank.co.uk/Technology/Inte rnet/Question407429.html
Chris