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After enabling the feature, Undo Send works much like Gmail’s other “undo” features. When you send an email, you get a message confirming it has been sent, along with a link to “Undo." This message lasts for 5 seconds, at which point you lose the opportunity to take it back.
While that might not be much time, it’s probably enough to pull back emails where you forget an attachment, forget to cc someone, or catch an obvious typo. As for emails you later wish you hadn’t sent because of the content, Gmail still can’t help you there.
More Gmail Resources From Mashable