On Saturday morning, one of Google's root certificates expired, causing millions of users' mail clients to suddenly protest. The certificate for Google's intermediate certificate authority (Google Internet Authority G2) was used to issue Gmail's certificate for SMTP, and the expiration at 11:55am EDT caused many e-mail clients to stop receiving Gmail messages. While the problem affected most Gmail users using PC and mobile mail clients, Web access to Gmail was unaffected.

Google reported on the company's Apps status page that engineers had been alerted to "issues with Gmail" at 1:21pm EDT on Saturday. In a later status update, a company spokesperson noted that "affected users are able to access Gmail but are seeing error messages and/or other unexpected behavior" and that "smtp.gmail.com is displaying an invalid certificate."

The root certificate for Google's certificate authority was issued by GeoTrust. By 4pm EDT Saturday, the certificate had been updated and is now valid until December 2016.