I always keep copies of all my sent emails in Mail.app from OS X Lion, at least since April 2010 (before I used Thunderbird). Today I was looking for some email I sent in September 2010 to some friend (I was sure about this, was about his birthday) but found nothing. Indeed, all email sent before December 2010 does not appear anymore in the "sent" folder.
I also remember in the last weeks, this old email on the "sent" folder, the subject was shown but could never read again the email message body.
PS: I remember some of the things that I have been activating lastly were "show messages by conversation" and sometimes I rebuild some of the folders