Is this happening all the time? if so try deleting your email account from Outlook and re entering it.
If its intermittent it could be a problem at Virgin Media's end and will sort it's self out. I had a similar problem the other day with Outlook 2007.
You could log onto your account via Virgin Media. Go to settings, forwarding POP/IMAP you should find links to help pages to check if your settings are correct.