Those ubiquitous Viagra ads have been disguising themselves as e-mail newsletters, the kind you get to find out the latest airline deals or keep up with your fantasy football team.

Spammers haven't actually broken into legitimate marketers' computer systems to send out the messages. Rather, like the phishing scams that lift the code off the real Web sites of financial institutions, spammers have tweaked legitimate e-mail and sent them through normal spam channels.

The technique appears aimed at bypassing human and software controls. Recipients might not immediately realize they are opening spam, and anti-spam filters might not be able to aggressively block them for fear of blocking legitimate newsletters as well, anti-spam experts say.

These messages started appearing a month ago, and so far, they have been relatively small in numbers, said Doug Bowers, senior director of anti-abuse engineering at Symantec Corp., a vendor of anti-spam products. He suspects spammers are fine-tuning their techniques to see what works.