I noticed a few days ago, what, in my opinion, is a serious bug in Mail. You see, I changed all my e-Mail addresses. Although I was able to maintain all my old inbox messages, I later realized that all my old "sent" messages had been unexpectedly deleted. There was no notification, no dialog box. They are all gone.
It decided to start from scratch with the messaged I've sent from my new accounts. I searched to see if they had been saved or archived in another folder in Mail, and found nothing. I suppose they may still exist, but they do not accessible via Mail.app. I find this destructive, or at least, apparently destructive (if the messages are still truly invisibly saved somewhere) behavior rather un-Apple like and unacceptable.