To get in on the un-sending action:
1. Go into your settings in Gmail
2. Scroll down, and check the box that says "Enable Undo Send"
3. Select a delay of anywhere between 5 and 30 seconds
4. Save changes
Now, every email you send will give you the option to "Undo" at the top of your inbox (next to the "View message" option) until the clock runs out on your pre-selected delay.