Google took a leap toward integrating Gmail and Google+ on Thursday as the company announced that you can now send an e-mail to anybody on Google+, even if you don't have their e-mail address.
The messaging option, which Gmail product manager David Nachum said will be rolling out to all Gmail users over the next few days, shares user names from Google+ in your Gmail contacts list, but not Gmail addresses. Those are only shared once the recipient has responded to your e-mail. The screenshot above shows how Gmail differentiates between Gmail contacts and Google+ contacts.
Basically, Google is allowing Google+ users to e-mail Gmail account owners, without actually learning what their Gmail address is until the person responds. E-mails received from Google+ users that you don't know will appear in your Primary Inbox category, but if the sender is not in your Circles, the e-mail will be filtered to your Social Inbox category. Google has created a Help page to assist people with the feature.
When you open the e-mail, you can choose whether or not to add the sender to your Circles. The person will be able to e-mail you only once, until you decide how to manage their e-mail. If you respond via Gmail, they obviously will learn your Gmail address. This, Google said, should cut down on spam and abusive e-mails.
Google has provided an opt-out measure, so that you can choose whether anyone on Google+ can e-mail you, or whether you want to restrict it to Extended Circles, Circles, or disable it entirely. Google+ users who are celebrities, at least on Google+, will have a different default setting. For them, their default setting will be people in their Circles, not everybody on Google+.
Nachum said that people will receive an e-mail in their Gmail inbox when the feature has been activated, which will include a link to your Gmail settings to disable it if you want. All new Google account users will receive the warning e-mail as well.
Given that Facebook has a similar option for messaging anybody who uses Facebook, it's not surprising that Google is moving its social network and e-mail services in a similar direction.
Update at 5:38 p.m. PT adds more details and a link to the feature's Help page.