According to security firm Marshal, spammers are using YouTube's servers and the firstname.lastname@example.org address to send massive quantities of unsolicited email, which appear to derive from the video-sharing site's 'Invite Your Friends' feature. The emails have the same appearance as a legitimate YouTube invite, except they include typical spam content and links to spam websites.
"YouTube users have a facility where they can invite their friends to view videos that they are looking at or have posted. This effectively allows them to email to any address from their YouTube account. This is the functionality that the spammers are exploiting. Spammers are doing this to defeat spam filters and to lower the recipient's guard by making it look as though the messages are coming from a perfectly innocuous email address. YouTube's own Help Center suggests that you exclude the email@example.com email address from spam filtering. The spammers are keenly aware of this," said Bradley Anstis, Marshal's director of product management.
News source: PC World