Yahoo has been rapped by consumer group Which for changing the terms and conditions of its email service to automatically scan messages.
Which came down on the mail and search company on Thursday. It said the changes allow Yahoo's email systems to automatically scan the content of emails.
"This is another example of creeping behavioural advertising, which we believe, is a gross violation of internet users' privacy," Georgina Nelson, an in-house lawyer for Which, said. "Most consumers would be horrified to learn that their email can be read in order to open the door to targeted advertisers."
Yahoo does this to help it fight spam and serve more targeted ads to users, it said. Gmail, the email service operated by Yahoo's search rival, Google, has been scanning messages to target ads since 2004.
Yahoo said it will ask for consent before scanning user emails.