Thirty-three percent of respondents said that they found it more difficult to determine what's a legitimate mailing.

According to an online survey of U.S. consumers by Harris Interactive, it is becoming harder for users to distinguish spam from legitimate e-mail. eSecurity Planet reports that the survey found that 33 percent of respondents said they found it more difficult to determine what's a legitimate mailing and what's a message from a scammer or rogue marketer.

Spam seems to still be working. Thirty-eight percent admitted they had at least once responded to spam messages. And of those who responded to spam, 55 percent were rewarded with either more spam or a computer virus.

Leon Rishniw, senior vice president of engineering for Cloudmark, which sponsored the study, says:

This survey demonstrates that there is still much work to be done to protect consumers from spam and the threats that it brings with it. Not only must users adopt the necessary technologies to fortify their defenses against e-mail threats, they also must be more vigilant about opening and responding to messages only from known senders.