The feature must be enabled by the user from Gmail's Settings screen, where the grace period can be set up to the maximum of 30 seconds.
Undo send was previously limited to Google Labs, the invite-only program where the Mountain View, Calif. company tests new features and prototypes before they're released to the general public. Late last month, Google added undo send to its Inbox by Gmail apps for Android and iOS.
When switched on, undo send does just that: Lets Gmail users recall a message. The only requirement is that the action must be taken within the grace period.
To turn on undo send in the Web-based Gmail, users should click on the gear icon at the upper right, select "Settings," then click on the General tab. The feature is listed about a third of the way down the page.