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When Gmail launched this feature, you only had 5 seconds to undo sending a message, but Google constantly added new periods: 10 seconds, 20 seconds and now 30 seconds. You can change the "send cancellation period" from the settings page.
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Probably a better idea would be to add an outbox, so that you can set a longer delay. Another benefit would be that you could also schedule sending an email message.
{ Thanks, Shimmy. }