Users of Google's Gmail service will soon be able to send messages directly to other Gmail accounts, regardless of whether the recipient has shared their email address.
The new feature will include everyone signed up to both Gmail and Google+, the web giant's social network.
The change has caused concern among some privacy advocates, who say people could be contacted by strangers.
Google says it will make it easier for users to communicate with friends.
But privacy campaigner Marc Rotenberg, of the Electronic Privacy Information Center, called the new feature "troubling", and compared it to a privacy flaw in a previous Google product.
"There is a strong echo of the Google Buzz snafu," he said, referring to an early incarnation of the company's social network, which initially made a list of Gmail users' contacts available publicly online.
'Easier to connect'
Gmail members will see a list of their Google+ connections when they begin to type a name in the 'To' field of their messages, even if they do not know the email address of the person they are attempting to contact.
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