Does Mail.app automatically delete old emails?

I just looked for an email from 2006 and it isn't there. In fact, nothing is in my Mail inbox past Nov 20, 2007. I'm pretty sure I didn't delete them, so does anyone know if Mail.app automatically delete emails from your inbox after a period of time? If so, how does one change this behavior (preferably to 'never').

 

I'm interested in the Inbox, not the others. I already have them set to how I want them. What's missing from that is the Inbox (and subfolders).

 

There is nothing about the Inbox in "Mailbox Behaviors".