Solved my own problem (again)
After rebuilding the system on Fedora 13x64 I had the same problem. /etc/httpd/conf.d contains specific configurations for various modules & software. In this case, mod_ssl & squirrelmail. The problem is that apache loads the configurations in the order they are listed in the directory (i.e. alphabetically), so ssl.conf was loaded after squirrelmail.conf (which depends on mod_ssl). The fix is to rename ssl.conf to mod_ssl.conf. It is also necessary to change ServerName to your main (or vitual) server's name. In my specific environment it was also necessary to open up port 443 on my router & to forward port 443 to the server on my local network.