Gmail has a very useful Labs feature called “undo send” that lets you “unsend” a message immediately after sending it. If you accidentally clicked on the “Send” button or you realize that you forgot to attach an important file, you have a few seconds to click on “undo” and continue to edit the message.
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.
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.