When using {tag_tellafriend} in email campaigns, the email that is sent from BC to the 'friend' is dysfunctional - the link rendered by {tag_itemurl} is redirected by a 302 Found to /_System/SystemPages/Error404, which simulates a Page Not Found error (it doesn't actually return 404).
Those links are in the format: http://www.example.com/CampaignProcess.aspx?A=View&KID=203100&VID=45681759