If your emails are going out to other domains then the problem is most likely occurring between Virgin Media and TalkTalk's servers.
It's even possible that Virgin Media are the victim of TalkTalk graylisting their servers, which may mean that the email will be delivered later or will eventually time out and you will get a message failed error in a few days time.
Virgin's Forum Team can escalate this for investigation, but it can take them up to 7 days to reach the thread.
Are you sending the emails via webmail or a mail program? If the latter - which?
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