In Yahoo

1) Once you have started to create your new e-mail, click on the button labeled "Save Draft."

2) The e-mail you are creating stays on the screen so you can continue to work. However, there is now a copy of what you're writing under DRAFTS. This is where you'll find the email again should something happen.  As you continue to write your email, click the "Save Draft" button as often as is necessary so that you keep as much of your email as possible, should something happen.