Some clients have highlighted that they have their messages rejected by ntlworld, and presumably other Virgin controlled email addresses.
I have seen this is a fairly common problem. I suspect it is to do with email signatures after testing, however it is impracticable to have all our clients adjust signatures to send to Virgin, as they are a large international group with many subsidiaries. There is also nothing that should deem them spam in these signatures.
Welcome to our forum, I am sorry you are having problems emailing our domains. We may reject emails as spam for many reasons. I am not sure if you are aware or not but we are presently taking steps to deliver suspected spam email to our customers and allow them to decide and then update our filters, more information here Spam Folder Settings Change
It may take us a few weeks to fully implement but once done fingers crossed it will sort your problem, if not do let us know and we will be happy to investigate further.