With the time people spend on e-mail each day, the number of messages can add up quickly. At least that’s what brothers Rambod and Sam Yadegar are betting on.

The pair recently launched GiveBackMail.com, a Westwood e-mail service that is sharing ad revenue with charities. It works this way: Each time a new ad appears when an e-mail is opened or sent, 25 percent of the revenue from that ad goes to a charity selected by the user.

GiveBackMail, which sells ads on its pages similar to those found on Yahoo Inc.’s service, lists seven different charities that users can choose among, including the American Red Cross and the Animal Welfare Association.

Chief Executive Rambod Yadegar acknowledged that a user will only end up directing a few dollars a year to his or her given charity, since most ads generate just 50 cents to $1.50 of revenue for every 1,000 impressions. But he said it can make a difference.

“The main overall concept is to have, say, 500,000 people just devoted to (donating to) cancer awareness,” he said. “Once you’re at a large scale like that, we’ll be able to donate millions a year.”

Since most users will not want to ditch their Yahoo or Gmail address in favor of an @givebackmail.com account, users can just forward messages to the site, which also allows people to manage multiple e-mail accounts.

GiveBackMail has attracted some 1,000 users since launching in May, but Lou Kerner, an analyst at downtown L.A.-based Wedbush Securities, said the site will struggle to attract users.

“You really need a fundamental improvement in the technology to get people to … change their behavior,” he said.