Release details for the product can be found on this page:
http://www.symantec.com/business/support/release_details.jsp?pid=51985
As you can see there are is currently no end of life date for the product. Saying that I don't believe there plans to add any new features and Symantec will only be fixing bugs. With your current license you can actually use the Symantec Brightmail Gateway. If I were you I would look to move to that product.
Kevin