The smtp.virginmedia.com server is designed for VM residential broadband customers to use. Use with any other ISP including Virgin Media business broadband is likely to either not work at all or lead to the problems identified in this thread. I believe HowardML is correct with his assessment of the situation and steps for resolution.
Thank you (and HowardML) for pointing that out - of course, the business broadband service is a separate entity. I missed that fact.
We are largely experts in VM's residential e-mail system. But what I can say is that if you are sending via Onetel's servers (as you see, now TalkTalk's servers) you need to ensure that your e-mail uses a from: address relating to onetel and you use TT's settings. - here: