If you ever send a Gmail message too early or you change your mind after you press "Send", there's a feature that will help you. It's called "Undo Send" and you can find in the crowded space of experimental features from Gmail Labs.
After enabling the feature, Gmail will show an "undo" link when you send a message. You have to react quickly because the link disappears in 5 seconds and there's no way to bring it back. If keyboard shortcuts are enabled in your Gmail account, a better option than clicking on "undo" is to press "z". When you undo sending a mail, Gmail saves it as a draft and you can continue editing the message or discard it.
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