A brand new feature is being tested in the GMail labs. Soon, you will be able to undo message sending, sent by mistake, or if you forgot to attach a file or include an important person(read boss!) in your email. Seems like GMail would be giving a tough fight to email clients like the Micrsoft Outlook. When you get all those features in your good old web browser, why would you wish to install a heavy program for it!!
The catch here is that you may undo send only within some stipulated time after you actually send a message, and that time being very short, only five seconds, I am not sure how effective would it be. But as far as my experience guides, I have observed that I catch mistakes in a sent message within a couple of seconds after clicking the SEND button. And even if you close the browser within those 5 seconds, your message will be sent anyways.
So, basically GMail holds on your message for 5 seconds after you send it. So if you observe any anomalies you can undo it. Impressive feature, but hope it does not add to the ever increasing downtimes in GMail.