Google is also ramping up its warnings for a rare type of Gmail intrusion: state-sponsored hacking attempts. These apply to fewer than 0.1 percent of Gmail users -- generally activists, journalists and policy-makers, Google says -- but they require immediate action. Now, suspected targets of these attacks will receive a full-page warning with instructions on how to protect their accounts. This is in addition to Google's existing warning, which places a red strip with a link to further information at the top of a suspected victim's Gmail page.

For Safer Internet Day in February, Google rolled out a new visual cue -- a broken red lock icon -- to alert users when they're corresponding with an account that doesn't support encryption. Since launching that feature, the amount of inbound mail sent using encryption has increased by 25 percent, Google says.