Stop messages from being sent for a few seconds after hitting the send button.
Step1: Login Gmail
Step2: Click to Settings LAB
Step3: Enable "Undo Send"
Step4: Final, Click to "Save Changes"
After your email Sent, a new Undo button on the Banner will display. You can click to that if you want to Undo send.
P/S: In this LAB Settings, you can Enable "Forgotten Attachment Detector"