Bright and early, this comes from Jim Hamm. "In Lion OS, first the bad news. Occasionally, for me anyway, the Mail App will quit unexpectedly, for no apparent reason. If, say, this happens after writing an email, perhaps with links and an image (ie., some work involved on my part), and before I can send it, it suddenly disappears as the app quits. Darn, what to do—start over?
"Now the good news. Apple, perhaps (hopefully) in the design engineers' wisdom, has included an auto-save feature in Lion. If I look in the Drafts Folder, there is my email, ready to be sent. And I hadn't even thought to save it as I typed the email. This auto-save is a nice, new feature in Lion.
Jim concludes, "A notice is automatically sent to Apple on the Mail failure so this bug can be corrected in a future OS update."