Aggressive efforts to protect e-mail users from spam are blocking many legitimate business messages from reaching consumers.

Return Path, a New York e-mail research firm with an engineering and marketing office in Superior, said Tuesday that Internet service providers are blocking as many as 37 percent of “opt-in” e-mails.

Those are messages that customers have agreed to receive, such as newsletters or airfare updates.

The company studied 30,000 e-mail marketing campaigns to arrive at its figures, said Jennifer Wilson, Return Path vice president of marketing.

More here.