Description of Problem:

The "This message has not been sent..." confirmation window only appears
when you close a compose window if you have modified the "To:" line or
"Subject:" line. 

Specifically (as I found out) you can spend 20 minutes writing a reply to
someone, accidentally hit "Ctrl-W" (which maps to close) and lose all of
your changes if you haven't modified the To: or Subject: lines.


Steps to reproduce the problem:
1. Reply to a message
2. Make changes to the message body - but NOT to the To: or Subject: lines
3. Close the window (either through the window manager, Ctrl-W or File->Close

Actual Results:

The window silently goes away, losing all of the changes you've made in the
message body


Expected Results:

You see a "The message has not been sent.  Do you wish to save your
changes?" popup

How often does this happen? 

Every time.