Well to your rescue Gmail recently added a small handy feature that can save your day.. Activating this feature would hold your email for few seconds after you hit send and you get an undo option to stop the transmission of the email.
Some would find this feature good to have and some whould say why delay the send email even for a sec. Well i would say the feature is optional to the user and infact google should allow the user to have control to set the time for hold.
To activate turn on Undo Send in Gmail Labs under Settings, and you’ll see a new “Undo” link on every sent mail confirmation for few secs. Click “Undo,” and google would grab the message before it’s sent and take you right back to compose.