If you are using Outlook you can recall the message.
If you are using Gmail you can turn on delayed sending and "undo" the send.
But if you are using Outlook as a frontend for Gmail what can you do?
One solution is to apply a delay before sending all emails. This can be accomplished by the following procedure:
- In Outlook, click Rules -> Manage Rules & Alerts.
- Click New Rule.
- Click Apply rule on message I send.
- Click Next.
- Click Yes in the window that asks if you want to apply it to all messages.
- Check the box next to defer delivery by a number of minutes.
- Click on a number of in the bottom window pane.
- Set the number of minutes that you want to delay by, e.g. 1 minutes.
- Click OK.
- Click Next.
- Give the rule a name, such as "Stop delivering stupid emails".
- Click Finish.
All emails that you send are now delayed by 1 minute. If you realize your mistake you can go into the Outbox and delete the email.
[Source: HowToGeek]