E-mail marketing messages account for 70% of spam complaints from consumers, according to a new report from Return Path Inc. The company, which certifies marketers’ e-mail as legitimate and monitors online reputations, adds that marketing e-mails make up only 18% of total e-mail volume.

Return Path based the findings in its third quarter “Email Intelligence Report” on an analysis of some 315,000 e-mail campaigns across the world. Return Path says that e-mail brings a return on investment of $40.56 per dollar spent, higher than the $22.24 for search and $19.72 for display ads.

The appeal of e-mail marketing comes in part from consumers signing up for numerous e-mail offers and mailing lists, the report says. But that tendency also contributes to spam problems for marketing e-mail. Return Path says consumers who sign up for e-mail messages often report as spam those messages that they no longer want to receive—oftentimes ignoring the option to unsubscribe.

“To control this problem, marketers need to confirm continued interest or remove people from their lists when they are no longer interested,” the report says.

Marketers also account for 60% of e-mails caught by spam traps, which are decoy accounts that Internet service providers set up to capture spam. Return Path blames that problem on marketers using e-mail addresses that consumers have abandoned, and the habit of some marketers of using e-mail addresses that have been scoured from the web or bought, instead of gained from consumer sign-ups.