Most of us have, at some time or other, sent an email only to realize that it contained something that was incomplete, misleading, or just plain wrong.
Gmail has a Lab to help you. The Undo Send lab can be added to Gmail via the cogwheel button, by selecting Settings from the drop-down menu and then Labs:
Once the Undo Send lab has been enabled (remember to save after enabling it), sending any email will cause the following message to pop up:
Simply click on the Undo link in this message to un-send the email message. You’ll receive another pop-up message confirming that Sending has been undone.
There’s one way you can customize the behavior of Undo Send. Back in the Settings page, choose the General tab and find the Undo Send entry:
Here you can change the time for which a sent email can be recalled. Once again, remember to save your changes if you modify this setting.
UPDATE: This feature has now graduated from Labs to become a feature of Gmail itself. If you had the Undo Send Lab enabled, the feature will automatically be enabled in Gmail. Otherwise, you can enable it in Settings.