E-mail marketers still need to monitor ISPs' e-mail blocking vigilantly, as the problem has worsened.

Twenty-two percent of permission-based e-mail was blocked by top ISPs in 2004, up 3.3 percent from the second half of 2003, e-mail management firm Return Path, New York, found in a new study.

"The spam market changes every day, and ISPs are changing how they're doing things every day," said Jennifer Wilson, vice president of marketing at Return Path. "You can't look at it once and then forget about it. You have to be monitoring it on an ongoing basis."

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