We all once have sent an email that we didn't want to sent. Either because we've sent it unfinished or we forgot to attach files. But gmail has a feature that will save you in such occasions.
In Gmail, go to your settings and enable Google Labs. The Undo Send feature allows you to stop messages from being sent for a few seconds after hitting the send button.
Another tip from us is to put recipient email address last. In other words, writing the recipient's email should be the last thing you do when sending an email.