Go to your
All mail view,
and click "Select all" (or the drop down option All from the checkbox far left of Archive | Spam | Delete buttons).
Underneath the | Archive | Spam | Delete | + | - | Move to Inbox | Labels | More |
buttons...
You'll see a message appear in a yellow bar that says:
All 100 conversations on this page are selected. Select all 73795 conversations in All Mail
Click on the "Select all conversations" link on the right side of the message, then click delete.
The actual deletion process may take a very, very long time, during which you will be unable to use Gmail or log in (don't worry, it's still churning away in the background, even if it gives you an error!)
When it's done, all conversations (except spam) will be in the trash and you should see a message that says "You don't have any mail! Our servers are feeling unloved."
Now just empty the trash and delete all spam, and your account is now empty.
It is now possible to label and/or "star" messages when you send them.
Official Gmail Blog: Star and label messages before you send them
When composing a new message, you can assign labels or star it by using the labels drop-down menu. As you’d expect, recipients of messages organized this way won’t see your labels or stars.
This is the new compose window:
Press the down arrow in the lower right, then choose "Label".
This is the old compose window:
Best Answer
There is an Undo Send lab which will give you an undo button as soon as you send a message. Unfortunately it won't help a past mistake, the email has already been send to the recipient's mail server.