Welcome to our Community, glad you've joined us for help with this
You should be able to send mail, for a 3rd party domain, via our SMTP servers. But the settings need to use your VM email details, the settings can be found here.
I suspect that your settings may be in order as you've used them with success previously.
One point of concern is that your VM email may be inactive on our systems - this will happen if you do not access for a period of 90 days. In that case you'd need to sign in to My Virgin Media and create a new email address - My Profile > Manage Accounts.
I'll send you a PM in a moment (purple envelope icon, top right of page) requesting details about your VM email so that I can check it out. Please reply when you can so we can get this sorted out