Today is the first i heard that google mail has an undo feature. I donno about you but I think I am going to be using that feature quite regularly. The reason being is that my instant reaction to things is not exactly the reaction I like to share.
Anyhow, here is a link
In short, go to Settings in Gmail --> Go to "Labs" tab and the look for (ctrl+f) Undo --> Enable the feature.
p.s. updating the blog after a long time cuz someone reminded me that this blog can be useful :) Thanks Danny