The company is trying out a new service known as CentMail. The site allows users to purchase packs of 500 virtual stamps at a cost of US$5, or one cent per stamp.
Proceeds from the sales are then given to a participating charity selected by the user, and each of the email stamps carries the logo of the charity.
In addition to benefitting charitable groups, Yahoo is also hoping that the program cuts down on spam. By adding the unique stamp to each message, the program is designed to breeze through spam filters.
"Anyone you email can automatically verify your donation and confirm you're not a spammer," the company wrote on the CentMail site.
"Since spammers send millions of emails every day, it is prohibitively expensive for them to donate even just 1 cent per email."
The move is one of many efforts to thwart the rising tide of spam email. Driven by botnets and online pharmacies, the global spam volume has climbed to new heights despite renewed efforts to shut down hosts and providers that harbour spammers.