Gmail users were seeing error messages and safety warnings recently after Google forgot to update a key part of the messaging software.
Google said a majority of its users were affected by the short-term software problem.
Many reported the errors using Twitter seeking clarification from Google about this glitch.
The error messages started appearing early on 4 April as people trying to send email messages from Gmail and some of the firm's messaging apps.
According to the BBC, the problems arose because Google had neglected to renew a security certificate for Gmail and its app services. The certificate helps the software establish a secure connection to a destination, so messages can be sent with little fear they will be spied upon.
Google's own in-house security service, called Authority G2, administers the security certificates and other secure software systems for Google.
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Google said its users were seeing error messages.
Google also said that the glitch could cause programs to act in unexpected ways.
The problem was resolved about two hours after it was first noticed.
The glitch comes soon after Google started refusing security certificates issued by the China Internet Network Information Center (CNNIC). Google said a security lapse by the CNNIC meant the certificates could no longer be trusted.