Gmail has a nifty little feature called “Undo Send” in Gmail Labs which allows you to unsend emails after you have sent them. You could unsend emails for up to 5 seconds until now, as Gmail delayed the emails for 5 seconds before it actually sent them. This gave Gmail an easy way to handle the huge email traffic flowing through its servers and also provided a nice feature to its users. It has helped me a lot of times by allowing me to unsend my emails and correct them in case there were any mistakes.
Google now allows you to dial the delay duration up to 30 seconds. You can change the “Send Cancellation Period” to 5, 10, 20 or 30 seconds by visiting Gmail > Settings > General > Undo Send.