How do you unsend an email?

For next time, desktop Outlook can hold every email for a minute or two before it actually goes, using a rule with the "defer delivery" action. Gmail has nothing like that built in apart from the 30 seconds, but scheduled send works if you want to sleep on something important.

Only for a few seconds, and only if you set it up beforehand. Gmail: the Undo link appears bottom left after you hit send. Default window is short, go to Settings > See all settings > General > Undo Send and bump it to 30 seconds, that's the max. Outlook.com has a similar undo send setting with a shorter window. The "Recall" option in desktop Outlook only really works inside the same company email system, and the other person can often still see the original. Once it has actually left, it's gone. There's no pulling it back from someone else's inbox.