Did you not receive a email from Virgin telling you about this. It is going on until the 24th and it is a free service. Pasted a bit below.
We're upgrading our email service very soon so, as part of a range of improvements, you'll automatically receive some anti-virus and email anti-spam protection on your Virgin.net email account. This will help reduce the amount of junk emails arriving in your Virgin.net email inbox.
Whether you're a Virgin.net Pay As You Go, 24seven or Broadband customer, these new services are totally free.