BMG sent out a rich media e-mail about Christina Aguilera’s new CD ‘Stripped,’ hoping to prompt tons of sales online two weeks before the album hit the stores last fall. But something else happened. The video clip of the gyrating, nearly naked pop star singing ‘Dirrty’ attracted a 40% pass-along rate – more pass-alongs than BMG had ever seen. Even better, 2,581 of the people who received the e-mail from a friend opted in to BMG’s Aguilera database. “You can’t get a more qualified lead,” said Kieve Huffman, senior director of strategic online marketing at New York-based BMG.