Adding to Gmail’s features destined to save you from embarrassment, Undo Send lets you abort the delivery of any Gmail message within moments of sending it. Undo Send can be activated from the Settings/Labs tab in Gmail.
Once the feature is activated, a new ‘Undo’ link will appear next to every sent e-mail confirmation. When clicked, ‘Undo’ will take you back to the composing window and confirm the retrieval of the e-mail.
However, ‘Undo Send’ doesn’t really recall a message that has been already sent. The feature holds e-mails in a queue for only five seconds, giving you time for second thoughts — though some already argue that this is not long enough.
Other ways not to send a message in Gmail include previous Labs features such as the Forgotten Attachment Detector (pinging the lack of an attachment if it was mentioned in the body) and Mail Googles (which asks you to answer simple math problems before sending e-mails).
Google also added on Wednesday four new Labs extensions that allow users to preview in their e-mail content from YouTube, Flickr, Picasa and Yelp.